[MIPS] Remove unused rm9k_cpu_irq_disable()

rm9k_cpu_irq_disable() is unused since commit
1603b5ac

.  Remove it.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>