Make the cylinder table columns unsortable

They were never intended to be sortable, but using common code with the
dive list picked up that "sort by index" thing by mistake.

If we really want to be able to sort cylinders by O2 percentage (which
really doesn't seem to make much sense, considering that you usually
have just one or two cylinders) we will need to also handle the case of
editing the (differently sorted) cylinder table.  Which we don't do now.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>