Bug 29200: Remove Adlibris cover service

Bug 29200: Remove Adlibris cover service

This patch removes integration of the Adlibris cover service which has
been discontinued.

To test, apply the patch and rebuild the OPAC and staff interface CSS:
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- Run the database update.
- Go to Administration -> System preferences -> Enhanced content. You
  should see no Adlibris-related preferences.
- Perform a catalog search and confirm that the search result and detail
  pages look correct.
- Enable one or more other cover image services and test that search
  results and detail page look correct.
- Perform the same test in the OPAC including these pages:
  - Catalog search results
  - Bibliographic detail page
  - Lists
  - Comments
  - User summary
  - User reading history

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>