compositor: destroy all input binding objects on shutdown

compositor: destroy all input binding objects on shutdown

wlsc_binding_destroy() was not being called at all. Fix the leaks by
introducing a function that destroys a whole list of bindings, instead
of individually saving an extra pointer to the binding object and
calling wlsc_binding_destroy() separately on each.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>