timekeeping: remove unused {read,update}_persistent_clock

After arch/sh has removed the last reference to these functions,
we can remove them completely and just rely on the 64-bit time_t
based versions. This cleans up a rather ugly use of __weak
functions.

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