nios2: Remove unused extern declaration of shm_align_mask

shm_align_mask is not defined/used on nios2, thus there is no need to
declare it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
nios2: Remove write-only struct member from nios2_timer

The irq member of struct nios2_timer is only written to but never read,
thus it can be removed.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>