Remove "preferSourceDebug" compile flag, use sourceDebugInformation

* This was previously only relevant for DXBC shaders that could have source
  debug information but didn't prefer source debug unless optimisation was
  disabled. This is inconsistent with how things are handled on SPIR-V and DXIL
  and in general it's considered now that the benefits of debugging in source
  outweigh the problems of debugging optimised code.

qrenderdoc/Windows/ShaderViewer.cpp

@@ -1105,16 +1105,7 @@ void ShaderViewer::debugShader(const ShaderReflection *shader, ResourceId pipeli
             m_Variables.push_back(c.after);
         }
 
-        bool preferSourceDebug = false;
-
-        for(const ShaderCompileFlag &flag : m_ShaderDetails->debugInfo.compileFlags.flags)
-        {
-          if(flag.name == "preferSourceDebug")
-          {
-            preferSourceDebug = true;
-            break;
-          }
-        }
+        bool preferSourceDebug = m_ShaderDetails->debugInfo.sourceDebugInformation;
 
         updateDebugState();