s390/compat: get rid of compat wrapper assembly code

Now that all compat syscalls have been converted to use the
COMPAT_SYSCALL_DEFINE macros, we don't need to compat syscall
wrapper assembly code anymore.
So remove it and fix up the system call table accordingly.

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