Bug 20157: Group search groups together

Bug 20157: Group search groups together

We do not actually need 2 groups, the previous feature worked for both
OPAC and STAFF interface.
The only difference was the "show_in_pulldown" feature.
Here we are going to REMOVE this feature for ergonomic reasons. We will
already have 3 features and it will surcharge the interface to add
another one. Moreover the feature will have to be applied at the OPAC
(and so will add lot of JS checks to keep data consistent: only useful
if ft_search_groups_opac is set).
Moreover it is quite easy to remove entry from the dropdown list in
JavaScript.

If people was really using this feature, we will re-add it, just let us
know.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>