fallback: cancel the arbitration timer on device remove

When the touch arbitration is reset to ARBITRATION_NOT_ACTIVE, the proximity
timer is set for 90ms to avoid erroneous touches (see 2a378bea for the
reason).

If the device is removed within those 90ms, the timer is never cancelled,
leading to an assert on cleanup.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>