Software backend: Delete forked PixelEngine.

Mostly just zapping a bunch of duplicated code; the only interesting thing
going on here is the changes to the performance counter implementation.

Source/Core/VideoCommon/MainBase.cpp

@@ -317,7 +317,3 @@ void VideoBackendHardware::RegisterCPMMIO(MMIO::Mapping* mmio, u32 base)
 	CommandProcessor::RegisterMMIO(mmio, base);
 }
 
-void VideoBackendHardware::RegisterPEMMIO(MMIO::Mapping* mmio, u32 base)
-{
-	PixelEngine::RegisterMMIO(mmio, base);
-}