rendering: remove builtin authoring register annotations

engine/assets/builtin/shaders/skybox/skybox.shader

@@ -9,7 +9,7 @@ Shader "Builtin Skybox"
         _Tex ("Cubemap (HDR)", Cube) = "white" [Semantic(SkyboxTexture)]
     }
     HLSLINCLUDE
-    cbuffer EnvironmentConstants : register(b0)
+    cbuffer EnvironmentConstants
     {
         float4 gSkyboxTopColor;
         float4 gSkyboxHorizonColor;
@@ -19,15 +19,15 @@ Shader "Builtin Skybox"
         float4 gCameraForwardAndUnused;
     };
 
-    cbuffer MaterialConstants : register(b1)
+    cbuffer MaterialConstants
     {
         float4 gSkyboxTintAndExposure;
         float4 gSkyboxRotationAndMode;
     };
 
-    Texture2D SkyboxPanoramicTexture : register(t0);
-    TextureCube SkyboxTexture : register(t1);
-    SamplerState LinearClampSampler : register(s0);
+    Texture2D SkyboxPanoramicTexture;
+    TextureCube SkyboxTexture;
+    SamplerState LinearClampSampler;
 
     struct VSOutput
     {