Remove the mechanism to comment out "attribute" as the COMPILEVS / COMPILEPS define mechanism already takes care of that, as long as it's also used for user-defined custom attributes. Fixes #870.

Source/Urho3D/Graphics/Shader.cpp

@@ -98,11 +98,10 @@ bool Shader::BeginLoad(Deserializer& source)
     CommentOutFunction(vsSourceCode_, "void PS(");
     CommentOutFunction(psSourceCode_, "void VS(");
 
-    // OpenGL: rename either VS() or PS() to main(), comment out vertex attributes in pixel shaders
+    // OpenGL: rename either VS() or PS() to main()
 #ifdef URHO3D_OPENGL
     vsSourceCode_.Replace("void VS(", "void main(");
     psSourceCode_.Replace("void PS(", "void main(");
-    psSourceCode_.Replace("attribute ", "// attribute ");
 #endif
 
     RefreshMemoryUse();