Bug 20443: Remove UpdateBorrowerAttribute and SetBorrowerAttributes

Bug 20443: Remove UpdateBorrowerAttribute and SetBorrowerAttributes

This patch replace Koha::Patron->get_extended_attributes with
->extended_attributes
It's now a getter a setter method.

It permits to replace UpdateBorrowerAttribute and use
create_related from DBIx::Class

Notes:
* We face the same variable names difference than in a previous patch
(value vs attribute)

Bug 20443: Remove SetBorrowerAttributes

squash  + RM get_extended_attributes

 RM get_extended_attributes

SQUASH Bug 20443: Remove UpdateBorrowerAttribute and SetBorrowerAttribute

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>