Removed GAMESCOPE_NV12_COLORSPACE default

This was being set in UpdateCompatEnvVars(), which is called after the environment variable is already read. The default in rendervulkan.cpp if the environment variable isn't set is BT.709 full-range, which is a sane default, so this isn't necessary.

src/main.cpp

@@ -611,9 +611,6 @@ static void UpdateCompatEnvVars()
 	// Don't minimise stuff on focus loss with SDL.
 	setenv( "SDL_VIDEO_MINIMIZE_ON_FOCUS_LOSS", "0", 1 );
 
-	// A sane default here.
-	setenv( "GAMESCOPE_NV12_COLORSPACE", "k_EStreamColorspace_BT601", 0 );
-
 	const char *pszMangoConfigPath = getenv( "MANGOHUD_CONFIGFILE" );
 	if ( (g_bLaunchMangoapp && steamMode) && ( !pszMangoConfigPath || !*pszMangoConfigPath ) )
 	{