Bug 34223: Work around ONLY_FULL_GROUP_BY in Illbackend

Bug 34223: Work around ONLY_FULL_GROUP_BY in Illbackend

This patch re-works the query in the existing_statuses method to remove
the FIXME and improve performance.  We pass an SQL literal into the
query to make it explicit which illrequest_id we're looking for (which
dampens the SQL warning) even though we really don't mind which request
is returned here.

Test plan:

The following command will (hopefully) reset your ILL data and create 10k fake ILL requests (run this in DEV KTD only).

1) On an empty k-t-d, run:
bash <(curl -s https://raw.githubusercontent.com/ammopt/koha-ill-dev/master/start-ill-dev-data.sh)
2) Pet a cat
3) Visit /cgi-bin/koha/ill/ill-requests.pl and select a backend on the left side filters
4) Notice how the status filter takes a while (3-5 secs) to load
5) Apply patch and koha-plack --restart kohadev
6) Repeat 3, notice how the status filter now loads fast

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Laura Escamilla <laura.escamilla@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>