sbin/mount_hammer: Remove duplicated MOPT_UPDATE macro definition

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 }