PAD: Call one joint pad header in most of pcsx2, not two. (#4985)

Add Gamepad.h

Cleanup.

Remove the Windows version of PAD.h as well.

Update CMakeLists.txt

Revert language changes.

pcsx2/SaveState.cpp

@@ -36,11 +36,7 @@
 #include "common/StringUtil.h"
 #include "SPU2/spu2.h"
 #include "USB/USB.h"
-#ifdef _WIN32
-#include "PAD/Windows/PAD.h"
-#else
-#include "PAD/Linux/PAD.h"
-#endif
+#include "PAD/Gamepad.h"
 
 #include <wx/wfstream.h>