s390: remove native mmap2() syscall

The mmap2() syscall has never been used on 64-bit s390x and should
have been removed as part of 5a79859a

 ("s390: remove 31 bit
support").

Remove it now.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>