MIPS: remove [SR]ocIt(2) IRQ handling code

This code is only invoked from Malta's corehi_irqdispatch function,
which would then promptly die() even if it did service an interrupt.
It most definitely isn't needed for any vaguely recent Malta system
and since it seems to be broken for any older ones & nobody even
noticed, just remove it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>