Bug 18928: (follow-up) Unset max_holds rule if needed

Bug 18928: (follow-up) Unset max_holds rule if needed

Test plan:
1) Set all values in Default checkout, hold and return policy section on
circulation rules admin page, try for all libraries and for one specific
library. Save.
2) Click on unset and confitm
--> without patch, the Maximum total holds allowed value remains
--> with patch the unset removex Maximum holds value too

Signed-off-by: Agustin Moyano <agustinmoyano@theke.io>
Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>