timekeeping: remove timespec_add/timespec_del

The last users were removed a while ago since everyone moved to ktime_t,
so we can remove the two unused interfaces for old timespec structures.

With those two gone, set_normalized_timespec() is also unused, so
remove that as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: John Stultz <john.stultz@linaro.org>