Bug 37787: Undocument koha-worker --queue elastic_index

Bug 37787: Undocument koha-worker --queue elastic_index

To test:
- Copy the koha-worker script to the standard location:
  $ sudo cp debian/scripts/koha-worker /usr/sbin/koha-worker
- Check "sudo koha-worker --help" and verify elastic_index is
  not mentioned in the list of "current queues"
- See https://wiki.koha-community.org/wiki/Testing_man_pages for
  how to check the "man" page. elastic_index should not be
  mentioned in the list of "Current queues".
- Both places aboce should have a note about elastic_index not
  being active when koha-es-indexer is running.

Signed-off-by: Sukhmandeep Benipal <sukhmandeep.benipal@inLibro.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>