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>