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>
(cherry picked from commit 4ed95f95ff)
Signed-off-by: Jacob O'Mara <jacob.omara@ptfs-europe.com>

koha-tmpl/intranet-tmpl/prog/en/modules/admin/preferences/patrons.pref

@@ -237,7 +237,7 @@ Patrons:
          - pref: BorrowerUnwantedField
            type: modalselect
            source: borrowers
-           exclusions: branchcode|categorycode
+           exclusions: branchcode|categorycode|flags
      -
          - "Borrowers can have the following titles:"
          - pref: BorrowersTitles