sbin/mount_hammer: Remove duplicated MOPT_UPDATE macro definition
include/mntopts.h has this for getmntopts(3).
mount_dirfs(8) has this too probably because it's been copy-pasted
from mount_hammer(8) though this commit doesn't touch that.
(the one in mount_devfs(8) is if0'd)
# grep MOPT_UPDATE . -riI | grep define
./contrib/smbfs/mount_smbfs/mntopts.h:#define MOPT_UPDATE { "update", 0, MNT_UPDATE, 0 }
./include/mntopts.h:#define MOPT_UPDATE { "update", 0, MNT_UPDATE, 0 }
./sbin/mount_devfs/mount_devfs.c:#define MOPT_UPDATE { "update", 0, MNT_UPDATE, 0 }
./sbin/mount_dirfs/mount_dirfs.c:#define MOPT_UPDATE { "update", 0, MNT_UPDATE, 0 }
./sbin/mount_hammer/mount_hammer.c:#define MOPT_UPDATE { "update", 0, MNT_UPDATE, 0 }