Re-enable eventhandler_deregister to vm_lowmem on arc_fini

Commit 622d8bba3 re-enabled code to register vm_lowmem event,
so the respective deregister should also be re-enabled.

'For every OSv specific ifdef we remove in ZFS, God ressurects a kitten.'
by Glommer.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>