Bug 16504: (follow-up for bug 15163) Do not remove attributes of other patrons

Bug 16504: (follow-up for bug 15163) Do not remove attributes of other patrons

Simple patch for a silly error, this single line is going to fix a
critical bug.
If a patron attribute is limited to a library, all the values for that attributes
for every patrons will be deleted.

Test plan:
Create a patron attribute limited to a library
Set the the attribute for a patron
Set the the attribute for another patron
=> Without this patch applied, the attribute's value for the first
patron is deleted
=> With this patch applied, the 2 values exist in the DB after the
second edition

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>