Removed Thickness parameter and set it to const value for ThinPlane mode

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Editor/LitUI.cs

-using System;
+using System;
 using UnityEngine;
 using UnityEngine.Rendering;
 using UnityEngine.Experimental.Rendering;
                 {
                     m_MaterialEditor.ShaderProperty(ior, Styles.refractionIORText);
 
-                    if (thicknessMap.textureValue == null)
-                        m_MaterialEditor.ShaderProperty(thickness, Styles.refractionThicknessText);
-                    m_MaterialEditor.TexturePropertySingleLine(Styles.refractionThicknessMapText, thicknessMap);
+                    if (mode != Lit.RefractionMode.ThinPlane)
+                    {
+                        if (thicknessMap.textureValue == null)
+                            m_MaterialEditor.ShaderProperty(thickness, Styles.refractionThicknessText);
+                        m_MaterialEditor.TexturePropertySingleLine(Styles.refractionThicknessMapText, thicknessMap);
-                    ++EditorGUI.indentLevel;
-                    m_MaterialEditor.ShaderProperty(thicknessMultiplier, Styles.refractionThicknessMultiplierText);
-                    --EditorGUI.indentLevel;
+                        ++EditorGUI.indentLevel;
+                        m_MaterialEditor.ShaderProperty(thicknessMultiplier, Styles.refractionThicknessMultiplierText);
+                        --EditorGUI.indentLevel;
+                    }
 
                     m_MaterialEditor.ShaderProperty(transmittanceColor, Styles.transmittanceColorText);
                     ++EditorGUI.indentLevel;

ScriptableRenderPipeline/HDRenderPipeline/Material/Lit/Lit.hlsl

-//-----------------------------------------------------------------------------
+//-----------------------------------------------------------------------------
 // SurfaceData and BSDFData
 //-----------------------------------------------------------------------------
 
     bsdfData.ior = ior;
     // Absorption coefficient from Disney: http://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf
     bsdfData.absorptionCoefficient = -log(transmittanceColor + 0.00001) / max(atDistance, 0.000001);
-    bsdfData.thickness = max(0.0001, thickness);
+#if defined(_REFRACTION_THINPLANE)
+    bsdfData.thickness = 0.03;
+#else
+    bsdfData.thickness = max(0.000001, thickness);
+#endif
 }
 
 // For image based lighting, a part of the BSDF is pre-integrated.