[S390] unwire sys_recvmmsg again

sys_recvmmsg is reachable via sys_socketcall. So unwire it again since
there is no point in having two entry points for it.
Also put it to the ignore list so we don't get reminded anymore in order
to wire it up.

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