um: Delete never executed timer

The "#ifdef undef" construction effectively disabled the timer.
It causes to the fact that this timer did nothing, so delete it.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>