s390/syscalls: remove SYSCALL_METADATA() from compat syscalls

SYSCALL_METADATA() is only supposed to be used for non-compat system
calls. Otherwise there would be a name clash.

This also removes the inconsistency that s390 is the only architecture
which uses SYSCALL_METADATA() for compat system calls, and even that only
for compat system calls without parameters. Only two such compat system
calls exist.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>