[S390] Use simple_strtoul instead of own cmm_strtoul wrapper.

Fix compile warning with some configurations:

arch/s390/mm/cmm.c:58: warning: 'cmm_strtoul' defined but not used

Originally cmm_strtoul was introduced because simple_strtoul couldn't
handle strings with hexadecimal numbers that contained a capital 'X'.
Since this is no longer true cmm_strtoul can be removed.

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