Bug 20456: Remove the C4::Serials::GetSubscriptionsFromBorrower

Bug 20456: Remove the C4::Serials::GetSubscriptionsFromBorrower

Bug 20400 added a routing list tab to the patron account in the
OPAC using Koha::Object. After changing the staff view too,
we can now safely remove GetSubscriptionsFromBorrower as it
is no longer used.

To test:
- Make sure that GetSubscriptionsFromBorrower is no longer used.

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>