Remove possible_crtcs and possible_clones workarounds

This is about the function drm_output_pick_crtc(): https://gitlab.freedesktop.org/wayland/weston/blob/78a42116ae92f93a01539a785ce95cc478189608/libweston/compositor-drm.c#L4809

It works around possible_crtcs and possible_clones being unreliable. If the kernel gets fixed to report realistic values, remove the workarounds.

However, if Weston gains support for atomic modesetting's TEST_ONLY commits and iterates over possible configurations to find one that works, then it might not be necessary to use the values, if not for optimizing out definitely invalid configurations.