Bug 27544: Simplify GET /checkouts implementation

Bug 27544: Simplify GET /checkouts implementation

This patch makes the list() controller method simpler, by removing the
checked_in parameter, and passing the appropriate restulset.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/checkouts.t
=> SUCCESS: Tests pass!
2. Apply this patch
3. Repeat (1)
=> SUCCESS: Tests still pass! (no behaviour change)
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>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>