rendering: remove builtin authoring register annotations

engine/assets/builtin/shaders/unlit/unlit.shader

@@ -18,7 +18,7 @@ Shader "Builtin Unlit"
             #pragma target 4.5
             #pragma vertex MainVS
             #pragma fragment MainPS
-            cbuffer PerObjectConstants : register(b0)
+            cbuffer PerObjectConstants
             {
                 float4x4 gProjectionMatrix;
                 float4x4 gViewMatrix;
@@ -26,13 +26,13 @@ Shader "Builtin Unlit"
                 float4x4 gNormalMatrix;
             };
 
-            cbuffer MaterialConstants : register(b1)
+            cbuffer MaterialConstants
             {
                 float4 gBaseColorFactor;
             };
 
-            Texture2D BaseColorTexture : register(t0);
-            SamplerState LinearClampSampler : register(s0);
+            Texture2D BaseColorTexture;
+            SamplerState LinearClampSampler;
 
             struct VSInput
             {