s390/time: remove select CLOCKSOURCE_VALIDATE_LAST_CYCLE again

Sven Schnelle reported that setting CLOCKSOURCE_VALIDATE_LAST_CYCLE
doesn't make sense: even if our tod clock overflows delta calculation
(now - last) with unsigned 64 bit values will still be correct.

Therefore revert commit 555701a7 ("s390/time: select
CLOCKSOURCE_VALIDATE_LAST_CYCLE").

Fixes: 555701a7 ("s390/time: select CLOCKSOURCE_VALIDATE_LAST_CYCLE")
Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>