Don't remove notify descriptor that is already gone

Don't remove notify descriptor that is already gone

* lisp/autorevert.el (auto-revert-use-notify, auto-revert-mode,
global-auto-revert-mode, auto-revert-notify-rm-watch,
auto-revert-notify-add-watch, auto-revert-notify-handler,
auto-revert-notify-rm-watch-callback):
Don't remove a notify descriptor after receiving a `stopped' notification
event, because the descriptor is then already gone and any attempt to
remove it causes a recursive call to `auto-revert-notify-handler'.