Remove sbatch --x11 option.

New X11 forwarding code will only support forwarding back to
salloc or an allocating srun command.

Using this option within sbatch was always hit-or-miss. If the
user submitting was disconnected from the alloc host for any
reason their xauth credentials would likely fail even if they
managed to get assigned the same local TCP port for forwarding.

Bug 3647.