timekeeping: Remove unused get_seconds()

The get_seconds() cleanup seems to have been completed, now it is
time to delete the legacy interface to avoid misuse later.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1606816351-26900-1-git-send-email-brookxu@tencent.com