fix($timeout): clean deferreds immediately after callback exec/cancel

Make sure $timeout callbacks are forgotten about immediately after
execution or cancellation.

Previously when passing invokeApply=false, the cleanup used $q and so
would be pending until the next $digest was triggered. This does not
make a large functional difference, but can be very visible when
looking at memory consumption of an app or debugging around the
$$asyncQueue - these callbacks can have a big retaining tree.