Remove fake screen modes that don't work

Prior to 9130b947, KWin generated common modes if there was only a single driver mode. It made no attempt to test that those modes are actually possible with the DRM KMS API, so users are presented with a list of fake modes that don't work on platforms that can't support them (like Apple systems).

That commit extended the generation to systems with more than one driver mode, further regressing things and now affecting even more systems (e.g. those with a single resolution but multiple refresh rates).

KWin can't just make up modes out of thin air. If the monitor claims it doesn't support certain modes, you can't just assume it does. Even if KMS allowed the commit through, the unsupported mode might not work with the actual monitor. This whole idea of injecting "common" modes by default seems deeply flawed. If anything, this should be a quirk limited to specific monitors/systems, or only enabled via a hidden config option.

I'm happy to send a patch for this, but I need to know what the original intent was here because this makes no sense to me on the face of it.