Bug 7317: (followup) Remove extra columns from backends on the main list

Bug 7317: (followup) Remove extra columns from backends on the main list

This patch removes the extra columns functionality, simplifying the code.
It removes redundant code (in both Perl and JS), unused vars.

It removes the use of here_link and hardcodes the script path on the template.

It also adjusts the AJAX call so it uses the 'library' param instead of 'branch'.

The library column now displays the library name instead of the ID.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>