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.

oktayacikalin/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,mammadori/pyglet,mammadori/pyglet,theblacklion/pyglet,theblacklion/pyglet,theblacklion/pyglet,mammadori/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,mammadori/pyglet,oktayacikalin/pyglet,oktayacikalin/pyglet