s390/stp: Remove leap second support

With moving time steering to userspace, there's no need
to handle leap seconds inside the kernel. Remove it.

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