Remove PrefersNonDefaultGPU from desktop file

[Jan200101]

PrefersNonDefaultGPU is currently very broken, see ValveSoftware/steam-for-linux#9940

initially added by #1052

[qreeves]

@Calinou You're probably more aware about the specifications than I, care to weigh in on this?

[qreeves]

I'll note that on Windows, this is accomplished with a DLL export that is specifically about NVIDIA Optimus/AMD Express:

#ifdef WIN32
// Force Optimus setups to use the NVIDIA GPU
extern "C"
{
#ifdef __GNUC__
__attribute__((dllexport))
#else
__declspec(dllexport)
#endif
    DWORD NvOptimusEnablement = 1;

#ifdef __GNUC__
__attribute__((dllexport))
#else
__declspec(dllexport)
#endif
    DWORD AmdPowerXpressRequestHighPerformance = 1;
}
#endif

[Calinou]

@Calinou You're probably more aware about the specifications than I, care to weigh in on this?

Yes, I agree it's best to remove it for now. Linux doesn't really have a good automatic solution for this just yet.