Bug 32535: Exclude flags from BorrowerUnwantedField system preference

Bug 32535: Exclude flags from BorrowerUnwantedField system preference

The system preference BorrowerUnwantedField includes a checkbox
for "System permissions (flags)." Checking it has no apparent effect;
one is still able to edit patron permissions with the box checked.
This patch excludes the field from the selectable fields.

To test:
* Search for the BorrowerUnwantedField system prefernce
* Verify that you can check/uncheck System permissions (flags)
* Verify it doesn't have any effect, as permissions are not set
  from the patron edit form, but separately
* Apply patch
* Verify you no longer can check/uncheck the field in the
  system preference

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>