timekeeping: remove obsolete time accessors

There are no more remaining users of these deprecated wrappers, so
let's remove them before new users have a chance to make it in.

See Documentation/core-api/timekeeping.rst for replacements when
porting old drivers that contain calls to this function.

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