Remove pending_swaps counter

As a protection against duplicate/early update times, it has already been replaced by commit 35aa2781 and commit 4faeb127.

Removing it also prevents a common cause of frame skips:

  clutter_stage_cogl_schedule_update()  
  clutter_stage_cogl_get_update_time() == -1  

Closes: gnome-shell#1411 (closed)