alarmtimer: Switch over to generic set/get/rearm routine

All required callbacks are in place. Switch the alarm timer based posix
interval timer callbacks to the common implementation and remove the
incorrect private implementation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211657.825471962@linutronix.de