Bug 17080: borrowers table - use default values defined in the DBIx::Class schema

Bug 17080: borrowers table - use default values defined in the DBIx::Class schema

This patch basically just revert bug 16960 - Fix error on validating the
registration

This job is now done by the Koha::Object constructor, no need to clean
the hash before sending it to AddMember.

Test plan:
Make sure there is no regression on bug 16960 (validate a self registration).

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>