s390: remove the ptep_modify_prot_{start,commit} exports

These two functions are only used by core MM code, so no need to export
them.

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