VideoCommon: Remove support for decoding to ARGB textures

The D3D / OGL backends only ever used RGBA textures, and the Software
backend uses its own custom code for sampling. The ARGB path seems to
just be dead code.

Since ARGB and RGBA formats are similar, I don't think this will make
the code more difficult to read or unable to be used as
reference. Somebody who wants to use this code to output ARGB can simply
modify the MakeRGBA function to put the shift at the other end.

Source/Core/VideoBackends/D3D/main.cpp

@@ -72,7 +72,6 @@ void InitBackendInfo()
 	}
 
 	g_Config.backend_info.APIType = API_D3D;
-	g_Config.backend_info.bUseRGBATextures = true; // the GX formats barely match any D3D11 formats
 	g_Config.backend_info.bUseMinimalMipCount = true;
 	g_Config.backend_info.bSupportsExclusiveFullscreen = true;
 	g_Config.backend_info.bSupportsDualSourceBlend = true;