m68k: Mac IRQ cleanup

There are no slow IRQs on Macs since Roman Zippel's IRQ reorganisation that
went into 2.6.16 and removed mac_irq_list[] and the do_mac_irq_list()
dispatcher. (They were implemented in do_mac_irq_list() by lowering the IPL.)
Hence there's no more use for mutual exclusion in the Mac interrupt
dispatchers. Remove it.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>