effects/contrast: Schedule contrast manager removal with a delay

This prevents flickering behind plasma panels after changing compositing
settings.

If the contrast effect is disabled, the destructor of the ContrastEffect will
start the contrast manager remove timer. When it expires, the contrast manager
global will be removed and it will be finally destroyed after 5s (to work
around the wl_global race condition).

If compositing is toggled, the destructor of the ContrastEffect will start
the remove timer and the constructor of the ContrastEffect will stop the
timer, the contrast manager object will be unaffected.

The contrast effect has no options that could affect the shader, so the
reconfigure() method was removed.