m68k/mac: cleanup mac_clear_irq

mac_clear_irq() is dead code and has been dead for as long as I can recall. On certain Mac models, certain irqs can't be cleared this way. Outside of irq dispatch, this code appears be unusable without busy loops or worse, and for irq dispatch we duplicate the same logic. Remove mac_clear_irq() and supporting code.

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