timekeeping: Remove unused timekeeping_{get,set}_tai_offset()

The last caller to timekeeping_set_tai_offset() was in commit
0b5154fb (timekeeping: Simplify tai updating from
do_adjtimex, 2013-03-22) and the last caller to
timekeeping_get_tai_offset() was in commit 76f41088 (hrtimer:
Cleanup hrtimer accessors to the timekepeing state, 2014-07-16).
Remove these unused functions now that we handle TAI offsets
differently.

Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>