clutter: Drop misguided assert

The clutter_seat_handle_event_post() function wants to handle
CLUTTER_DEVICE_ADDED/REMOVED to perform signal emission, but
checks (and asserts) that every event going through it has a
source device.

This is no longer quite true for IM events (they are attached
to the ClutterSeat's keyboard, not a HW device), so the assert
can now fire off (of course undesiredly).

But anyways, for events built through
clutter_event_device_notification_new() (the ones this function
is interested in, after all), it is already a precondition check
that the device is proper at the time of creating the event, so
asserting for it here is redundant.

We can drop this overly generic assert, this is already ensured
for the events that matter, anyways.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3167>

clutter/clutter/clutter-seat.c

@@ -639,7 +639,6 @@ clutter_seat_handle_event_post (ClutterSeat        *seat,
     seat_class->handle_event_post (seat, event);
 
   device = clutter_event_get_source_device (event);
-  g_assert_true (CLUTTER_IS_INPUT_DEVICE (device));
 
   switch (clutter_event_type (event))
     {