[MIPS] Terminally fix local_{dec,sub}_if_positive

They contain 64-bit instructions so wouldn't work on 32-bit kernels or
32-bit hardware.  Since there are no users, blow them away.  They
probably were only ever created because there are atomic_sub_if_positive
and atomic_dec_if_positive which exist only for sake of semaphores.

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