um: Remove misleading #define ARCh_IRQ_ENABLED

Due to the typo in the name, this can never be used, but
it's also misleading because our value for enabled/disabled
is always just 0/1, not an actual signal mask.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>