Revert "gdk/frameclock: add mark when we discover frames may drop"

This reverts commit 84a304e6.

This produces marks that are confusing to me. They don't correlate with actual gaps in the frame cycle and often overlap with regular 'window presented' marks. Also, the function we are emitting these marks from is called from the get_frame_time getter, and we definitely don't want to emit marks from there.