timekeeping: Eliminate the stale declaration of ktime_get_raw_and_real_ts64()

Commit ba26621e got rid of ktime_get_raw_and_real_ts64(), but left its
declaration behind.

Remove it.

Fixes: ba26621e ("time: Remove duplicated code in ktime_get_raw_and_real()")
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Christopher S. Hall <christopher.s.hall@intel.com>
Cc: joelaf@google.com
Cc: arnd@arndb.de
Cc: gregkh@linuxfoundation.org
Cc: john.stultz@linaro.org
Cc: deepa.kernel@gmail.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/1510552144-20831-1-git-send-email-douly.fnst@cn.fujitsu.com