[S390] Get rid of ARCH_KMALLOC_MINALIGN.

slab cache debugging code has been changed so that we always get a
minimum alignment of the alignment of a 64-integer. Since this is
8 on s390/s390x there is no need of ARCH_KMALLOC_MINALIGN anymore.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>