WIP: clutter/stage-cogl: 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As a protection against duplicate/early update times, it has already been replaced by commit 35aa2781 and commit 4faeb127.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)

This landed already as !719 (merged) but then got reverted in 97140ab6 due to regression #735 (closed). This new version includes a fix for #735 (closed) to stop that happening again.