Retire wx_syscall regress tests, they have been failing since the

Retire wx_syscall regress tests, they have been failing since the
introduction of mimmutable(2) and also needs to cope with xonly on some
architectures by now. There's already the syscallwx target in
regress/usr.bin/lastcomm which does the same thing but also ensures that
the expected code path is reached by looking at accounting records.