s390/time: Remove in-kernel time steering

Remove the in-kernel time steering in favour of the new
ptp s390 driver, which allows the kernel clock to be steered
more precise.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>