Bug 35570: Prevent use of ILL module if deprecated FreeForm backend is installed

Bug 35570: Prevent use of ILL module if deprecated FreeForm backend is installed

Test plan:
1) This patch should not be applied in order to be able to go through the previous patch's test plan
2) Install FreeForm, run:
bash <(curl -s https://raw.githubusercontent.com/ammopt/koha-ill-dev/master/start-ill-dev.sh)
3) Please note that you need to be using FreeForm's reorganize_ILL branch at:
https://github.com/PTFS-Europe/koha-ill-freeform/commits/reorganize_ILL/
4) Visit ILL page:
/cgi-bin/koha/ill/ill-requests.pl
5) Notice there's a configuration error preventing the use of the ILL module.
6) Click the 'about page' link and verify there is a warning about
   FreeForm
7) Remove FreeForm from /kohadevbox/koha/Koha/Illbackends and refresh the page
8) Repeat 4), notice the requests are listed as expected and you can now
   use the ILL module again.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>