Bug 18427: Add a primary key to serialitems

Bug 18427: Add a primary key to serialitems

We drop the unique index serialitemsidx and add a primary key consisting
of itemnumber just as the unique key did.

Test plan:
Run the db revision.
Apply bug 18182. The TestBuilder.t should no longer fail on serialitems.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>