time: x86: Remove CLOCK_TICK_RATE from mach_timer.h

CLOCK_TICK_RATE is defined as PIT_TICK_RATE on x86 so we
update mach_timers.h to just use the later as we want
to depecrate CLOCK_TICK_RATE.

Signed-off-by: Deepak Saxena <dsaxena@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>