effects/blur: Schedule blur manager removal with a delay

This prevents flickering behind plasma panels after changing compositing
settings.

If the blur effect is disabled, the destructor of the BlurEffect will
start the blur manager remove timer. When it expires, the blur 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 BlurEffect will start
the remove timer and the constructor of the BlurEffect will stop the
timer, the blur manager object will be unaffected.

The blur effect options don't affect the generated shader code, so the
shader status check was removed in reconfigure().