timekeeping: Remove stale function prototype

commit 61edec8 "timekeeping: Simplify timekeeping_clocktai()"
implemented timekeeping_clocktai() as an inline function, but left the
old extern prototype in the header file. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/timekeeping.h

@@ -15,7 +15,6 @@ extern u64 timekeeping_max_deferment(void);
 extern int timekeeping_inject_offset(struct timespec *ts);
 extern s32 timekeeping_get_tai_offset(void);
 extern void timekeeping_set_tai_offset(s32 tai_offset);
-extern void timekeeping_clocktai(struct timespec *ts);
 extern int timekeeping_suspend(void);
 extern void timekeeping_resume(void);