Fix memory leak in clock.  Reverse function->item mapping was not being cleaned up; exacerbated by sprite's many one-shot timers.  Removed whole mapping, just search for items during unschedule() instead, it's an uncommon use-case.
Fix memory leak in clock.  Reverse function->item mapping was not being
cleaned up; exacerbated by sprite's many one-shot timers.  Removed whole
mapping, just search for items during unschedule() instead, it's an uncommon
use-case.

kmonsoor/pyglet,Alwnikrotikz/pyglet,kmonsoor/pyglet,mpasternak/michaldtz-fix-552,mpasternak/pyglet-fix-issue-518-522,xshotD/pyglet,mpasternak/pyglet-fix-issue-552,arifgursel/pyglet,arifgursel/pyglet,Austin503/pyglet,odyaka341/pyglet,shaileshgoogler/pyglet,Alwnikrotikz/pyglet,mpasternak/pyglet-fix-issue-518-522,shaileshgoogler/pyglet,arifgursel/pyglet,arifgursel/pyglet,cledio66/pyglet,shaileshgoogler/pyglet,Alwnikrotikz/pyglet,mpasternak/michaldtz-fix-552,mpasternak/pyglet-fix-issue-552,mpasternak/michaldtz-fixes-518-522,gdkar/pyglet,shaileshgoogler/pyglet,xshotD/pyglet,kmonsoor/pyglet,mpasternak/pyglet-fix-issue-552,mpasternak/pyglet-fix-issue-518-522,gdkar/pyglet,odyaka341/pyglet,qbektrix/pyglet,cledio66/pyglet,Austin503/pyglet,Austin503/pyglet,google-code-export/pyglet,mpasternak/michaldtz-fixes-518-522,shaileshgoogler/pyglet,google-code-export/pyglet,arifgursel/pyglet,xshotD/pyglet,qbektrix/pyglet,kmonsoor/pyglet,xshotD/pyglet,odyaka341/pyglet,mpasternak/pyglet-fix-issue-552,qbektrix/pyglet,odyaka341/pyglet,mpasternak/michaldtz-fix-552,cledio66/pyglet,mpasternak/michaldtz-fix-552,cledio66/pyglet,Austin503/pyglet,google-code-export/pyglet,cledio66/pyglet,gdkar/pyglet,mpasternak/pyglet-fix-issue-518-522,Alwnikrotikz/pyglet,google-code-export/pyglet,Austin503/pyglet,Alwnikrotikz/pyglet,qbektrix/pyglet,gdkar/pyglet,xshotD/pyglet,google-code-export/pyglet,gdkar/pyglet,kmonsoor/pyglet,odyaka341/pyglet,mpasternak/michaldtz-fixes-518-522,qbektrix/pyglet,mpasternak/michaldtz-fixes-518-522