Bug 32544: Remove flags as option from several patron form related system preferences

Bug 32544: Remove flags as option from several patron form related system preferences

Because permissions aren't actually set via the patron edit
screens, these checkboxes have no real purpose or effect.

This removes flags from the selectable fields of the following
system preferences:

* PatronSelfModificationMandatoryField
* PatronSelfRegistrationBorrowerMandatoryField
* BorrowerMandatoryField
* PatronSelfModificationBorrowerUnwantedField
* PatronSelfRegistrationBorrowerUnwantedField

To test:
* Apply patch
* Verify that the checkbox for flags can no longer be set for
these preferences

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomás Cohen Arazi <tomascohen@theke.io>