Bug 15142: Remove titles facet for UNIMARC

Bug 15142: Remove titles facet for UNIMARC

There is a facet using 500$a, 501$a and 503$a and Subject index.
In UNIMARC 500$a, 501$a and 503$a are not indexed as Subject.
I propose to remove this facet for UNIMARC, looks like it does not make
sens.

Test plan:
1) Install UNIMARC database
2) Edit some records to have 500$a
3) Perform a search to find those records
4) Check you dont see the titles facet

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>