Bug 9988: Remove the Zebra code from sub merge

Bug 9988: Remove the Zebra code from sub merge

Since we can now call linked_biblionumbers, we can now remove all Zebra
related code from merge. We also add a parameter biblionumbers; we use it
in the test now, but it may be handy too later in the maintenance script
when we want to trigger a merge for specific biblionumber(s). See bug
report 18071.

All mocks for ZOOM, Context::Zconn, Search::new_record_for_zebra in the
merge test can now be replaced by one mock for linked_biblionumbers. Note
that we test the biblionumbers parameter in the last subtest without that
mock.

Remove unused vars $countunmodifiedbiblio, $counterrors from merge.
Renamed zebrarecords to linkedrecords in the Merge test.

Test plan:
[1] Run t/db_dependent/Authorities/Merge.t
[2] Modify an authority record. Check the linked biblio records.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>