Bug 25601: (bug 18928 follow-up) Fix "Unset" for "return policy for $branch"

Bug 25601: (bug 18928 follow-up) Fix "Unset" for "return policy for $branch"

returnbranch has to pass an itemtype key when setting the rule (here
setting it to unlimited for any item types)

Test plan:
1. Go the "Circulation and fine rules" admin page
2. Set a rule for "Default checkout, hold and return policy"
3. Unset it
4. Select a library
5. Set a rule for "Default checkout, hold and return policy"
6. Unset it

Display should be consistent with what you selected

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit f1cc9d6b599ae423d8024fb11f3f601d57ab9963)
Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>