Bug 33671: Update POD, remove unneeded check in relationship->guarantor

Bug 33671: Update POD, remove unneeded check in relationship->guarantor

[1] Update POD for $patron->guarantor_relationships
[2] Remove check from Patron::Relationship->guarantor

Test plan:
Run t/db_dependent/Koha/Patron.t
Run t/db_dependent/Patron/Relationships.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 5eaee314c59a4ed8f3d75c7897f931f57f1577b1)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>