Revert "s390/smp,ptdump: add absolute lowcore markers"

This reverts commit e409b7f1.

Commit 7d06fed7 ("s390/smp: rework absolute lowcore access")
introduced mutex lock with interrupts disabled. This commit is
a follow-up that needs to be reverted as well.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>