[MIPS] Sibyte: Delete {sb1250,bcm1480}_steal_irq().

They break the timer interrupt initialization and only seem to be a kludge
for initialization happening in the wrong order.  Further testing done by
Thiemo confirms the suspicion that the other invocations also seem to have
useless.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>