Bug 17080: borrowers.checkprevcheckout - use the default value defined in the DBIx...

Bug 17080: borrowers.checkprevcheckout - use the default value defined in the DBIx::Class schema

This line has been added by bug 16917 to define the default value of
borrowers.checkprevcheckout.
As this value is defined in the DBIx::Class schema, we should not have
to handle it here.

Test plan:
Confirm that this patch does not reintroduced the bug fixed by bug
16917.

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>