Bug 24368: Remove Koha::Libraries->pickup_locations

Bug 24368: Remove Koha::Libraries->pickup_locations

This patch removes the unused method, and cleans the tests.
To test:
1. Verify Koha::Libraries->pickup_locations is not used in the code:
   $ git grep 'Koha::Libraries\->pickup_locations'
=> SUCCESS: no uses
2. Apply this patch
3. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Libraries.t
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>