[MIPS] time: Delete dead cycles_per_jiffy, mips_timer_ack and null_timer_ack

cycles_per_jiffy was only ever getting assigned and the function pointer
not being called anymore and mips_timer_ack had gotten similarly stale.  I
leave the remaining assignments unfixed as a lighthouse pointing platform
maintainers to what needs a rewrite.  These changes make null_timer_ack()
unreferenced, so delete that too.

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