Bug 16909: Koha::Patrons - Remove checkuniquemember

Bug 16909: Koha::Patrons - Remove checkuniquemember

C4::Members::checkuniquemember was not really nicely written, was only
used once and was not covered by tests.
I think it does not make sense to keep such complexity and have this
code in the subroutine/method.
Looking at this patch it seems that what this subroutine did can be done
easily in the pl script in few lines.

Test plan:
1/ Create 2 organisations with the same "surname": you should get a
warning.
2/ Create 2 patrons (non-organisation) with the same
surname/firstname/date of birth, you should get a warning

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>