[POWERPC] Delete unused irq functions on powerpc

The ack_irq macro is unused and conflicts with James' work to template
the generic irq code.  mask_irq and unmask_irq are also unused, so delete
those macros too.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Paul Mackerras <paulus@samba.org>