Bug 5373 - allow 'cardnumber' to appear on the patron import field list

Bug 5373 - allow 'cardnumber' to appear on the patron import field list

For some reason, it was explicitly removed, however you need it if you
want to match up on cardnumber.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Easy to test and works nicely.
Before patch cardnumber is not shown in the list of fields on the
patron import page. After applying the patch it is.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>