[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>

include/asm-mips/time.h

@@ -40,7 +40,6 @@ extern int rtc_mips_set_mmss(unsigned long);
  * mips_timer_ack may be NULL if the interrupt is self-recoverable.
  */
 extern int (*mips_timer_state)(void);
-extern void (*mips_timer_ack)(void);
 
 /*
  * High precision timer clocksource.