posix-timers: Remove unused export of posix_timer_event()

Since the removal of the mmtimer driver the export is not longer needed.
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/20170530211656.052744418@linutronix.de

kernel/time/posix-timers.c

@@ -442,7 +442,6 @@ int posix_timer_event(struct k_itimer *timr, int si_private)
 	/* If we failed to send the signal the timer stops. */
 	return ret > 0;
 }
-EXPORT_SYMBOL_GPL(posix_timer_event);
 
 /*
  * This function gets called when a POSIX.1b interval timer expires.  It