Remove Double Sided mode.

com.unity.render-pipelines.high-definition/HDRP/Editor/Material/Lit/BaseLitUI.cs

         {
             base.FindBaseMaterialProperties(props);
 
-            doubleSidedNormalMode = FindProperty(kDoubleSidedNormalMode, props);
+            doubleSidedNormalMode = FindProperty(kDoubleSidedNormalMode, props, false);
             depthOffsetEnable = FindProperty(kDepthOffsetEnable, props, false);
 
             // MaterialID
             EditorGUI.indentLevel++;
 
             // This follow double sided option
-            if (doubleSidedEnable.floatValue > 0.0f)
+            if (doubleSidedEnable != null && doubleSidedEnable.floatValue > 0.0f)
             {
                 EditorGUI.indentLevel++;
                 m_MaterialEditor.ShaderProperty(doubleSidedNormalMode, StylesBaseLit.doubleSidedNormalModeText);
         {
             SetupBaseUnlitKeywords(material);
 
-            bool doubleSidedEnable = material.GetFloat(kDoubleSidedEnable) > 0.0f;
-
-            if (doubleSidedEnable)
+            if (material.HasProperty(kDoubleSidedEnable) && material.GetFloat(kDoubleSidedEnable) > 0.0f)
             {
                 DoubleSidedNormalMode doubleSidedNormalMode = (DoubleSidedNormalMode)material.GetFloat(kDoubleSidedNormalMode);
                 switch (doubleSidedNormalMode)

com.unity.render-pipelines.high-definition/HDRP/Material/TerrainLit/TerrainLit.shader

         [HideInInspector] _ZTestDepthEqualForOpaque("_ZTestDepthEqualForOpaque", Int) = 4 // Less equal
         [HideInInspector] _ZTestGBuffer("_ZTestGBuffer", Int) = 4
 
-        [ToggleUI] _DoubleSidedEnable("Double sided enable", Float) = 0.0
-        [Enum(Flip, 0, Mirror, 1, None, 2)] _DoubleSidedNormalMode("Double sided normal mode", Float) = 1
-        [HideInInspector] _DoubleSidedConstants("_DoubleSidedConstants", Vector) = (1, 1, -1, 0)
-
         [ToggleUI] _EnableInstancedPerPixelNormal("Instanced per pixel normal", Float) = 1.0
 
         // Caution: C# code in BaseLitUI.cs call LightmapEmissionFlagsProperty() which assume that there is an existing "_EmissionColor"
     #pragma shader_feature _MASKMAP
     // Sample normal in pixel shader when doing instancing
     #pragma shader_feature _TERRAIN_INSTANCED_PERPIXEL_NORMAL
-
-    #pragma shader_feature _DOUBLESIDED_ON
 
     //#pragma shader_feature _ _LAYER_MAPPING_PLANAR0 _LAYER_MAPPING_TRIPLANAR0
     //#pragma shader_feature _ _LAYER_MAPPING_PLANAR1 _LAYER_MAPPING_TRIPLANAR1

com.unity.render-pipelines.high-definition/HDRP/Material/TerrainLit/TerrainLitData.hlsl

     // terrain lightmap uvs are always taken from uv0
     input.texCoord1 = input.texCoord2 = input.texCoord0;
 
-    ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
-
     float3 normalTS;
     TerrainSplatBlend(input.texCoord0, input.worldToTangent[0], input.worldToTangent[1],
         surfaceData.baseColor, normalTS, surfaceData.perceptualSmoothness, surfaceData.metallic, surfaceData.ambientOcclusion);

com.unity.render-pipelines.high-definition/HDRP/Material/TerrainLit/TerrainLitData_Basemap.hlsl

     // terrain lightmap uvs are always taken from uv0
     input.texCoord1 = input.texCoord2 = input.texCoord0;
 
-    ApplyDoubleSidedFlipOrMirror(input); // Apply double sided flip on the vertex normal
-
     surfaceData.baseColor = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, input.texCoord0).rgb;
 
     surfaceData.perceptualSmoothness = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, input.texCoord0).a;

com.unity.render-pipelines.high-definition/HDRP/Material/TerrainLit/TerrainLitSharePass.hlsl

 #define VARYINGS_NEED_TANGENT_TO_WORLD
 #define VARYINGS_NEED_TEXCOORD0
 
-#ifdef _DOUBLESIDED_ON
-    #define VARYINGS_NEED_CULLFACE
-#endif
-
 #if defined(UNITY_INSTANCING_ENABLED) && defined(_TERRAIN_INSTANCED_PERPIXEL_NORMAL)
     #define ENABLE_TERRAIN_PERPIXEL_NORMAL
 #endif

com.unity.render-pipelines.high-definition/HDRP/Material/TerrainLit/TerrainLit_Basemap.shader

         [HideInInspector] _ZTestDepthEqualForOpaque("_ZTestDepthEqualForOpaque", Int) = 4 // Less equal
         [HideInInspector] _ZTestGBuffer("_ZTestGBuffer", Int) = 4
 
-        [ToggleUI] _DoubleSidedEnable("Double sided enable", Float) = 0.0
-        [Enum(Flip, 0, Mirror, 1, None, 2)] _DoubleSidedNormalMode("Double sided normal mode", Float) = 1
-        [HideInInspector] _DoubleSidedConstants("_DoubleSidedConstants", Vector) = (1, 1, -1, 0)
-
         // Caution: C# code in BaseLitUI.cs call LightmapEmissionFlagsProperty() which assume that there is an existing "_EmissionColor"
         // value that exist to identify if the GI emission need to be enabled.
         // In our case we don't use such a mechanism but need to keep the code quiet. We declare the value and always enable it.
     #pragma target 4.5
     #pragma only_renderers d3d11 ps4 xboxone vulkan metal
 
-    #pragma shader_feature _DOUBLESIDED_ON
     #pragma shader_feature _DISABLE_DBUFFER
     #pragma shader_feature _TERRAIN_INSTANCED_PERPIXEL_NORMAL
 

com.unity.render-pipelines.high-definition/HDRP/RenderPipelineResources/DefaultHDTerrainMaterial.mat

         m_Offset: {x: 0, y: 0}
     m_Floats:
     - _CullMode: 2
-    - _DoubleSidedEnable: 0
-    - _DoubleSidedNormalMode: 1
     - _EnableInstancedPerPixelNormal: 1
     - _HeightTransition: 0
     - _LayerBlendMode: 0
     - _ZWrite: 1
     m_Colors:
     - _Color: {r: 1, g: 1, b: 1, a: 1}
-    - _DoubleSidedConstants: {r: 1, g: 1, b: -1, a: 0}
     - _EmissionColor: {r: 1, g: 1, b: 1, a: 1}