s390/maccess: remove dead DAT-off code

As the kernel is executed in DAT-on mode only, remove
unnecessary DAT bit check together with the dead code.

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