Properly disable mixed thickness mode for directional lights

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.hlsl

     float  NdotL = dot(N, L);
     float  LdotV = dot(L, V);
 
-    bool mixedThicknessMode = HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_MIXED_THICKNESS);
-
-    UNITY_BRANCH
-    if (mixedThicknessMode && NdotL < 0 && lightData.shadowIndex >= 0)
-    {
-        // Make sure we do not sample the shadow map twice.
-        lightData.shadowIndex = -1;
-    }
-
     float3 color;
     float attenuation;
     EvaluateLight_Directional(lightLoopContext, posInput, lightData, bakeLightingData, N, L, color, attenuation);
         lighting.specular *= intensity * lightData.specularScale;
     }
 
+    // The mixed thickness mode is not supported by directional lights due to poor quality and high performance impact.
+    bool mixedThicknessMode = HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_MIXED_THICKNESS);
+
-    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIALFEATUREFLAGS_LIT_TRANSMISSION))
+    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIALFEATUREFLAGS_LIT_TRANSMISSION) && !mixedThicknessMode)
     {
         // We use diffuse lighting for accumulation since it is going to be blurred during the SSS pass.
         lighting.diffuse += EvaluateTransmission(bsdfData, bsdfData.transmittance, NdotL, NdotV, LdotV, attenuation * lightData.diffuseScale);