Fix various warnings reported by clang

- mostly remove unused variables
- rename some generic JIT identifiers

Source/Core/VideoCommon/BPFunctions.cpp

@@ -14,7 +14,6 @@
 #include "VideoCommon/VertexShaderManager.h"
 #include "VideoCommon/VideoConfig.h"
 
-const bool renderFog = false;
 namespace BPFunctions
 {
 // ----------------------------------------------