Bug 18149: Move CountUsage calls to Koha namespace

Bug 18149: Move CountUsage calls to Koha namespace

After the introduction of Koha::Authorities->get_usage_count with bug
9988, we can now replace the remaining occurrences of CountUsage.

At the same time we remove CountUsageChildren. This was an empty sub.
The typo get_count_usage in a subtest title is adjusted.

Test plan:
[1] Run t/db_dependent/Koha/Authorities.t
[2] Perform a search on authorities-home.pl and verify that you see
    plausible numbers for 'used in xx records'.
[3] Click on Details for one authority. See the same number?
[4] Do the same as in 2/3 for Authority search on OPAC.
[5] Remember the authid and enter this in the record numbers box on
    tools/batch_delete_records.pl. Select Authorities and click
    Continue. The next form shows a column "Used in". Do you see
    the same count again?
[6] Git grep CountUsage.
    You should see just one hit in a comment that can be kept in
    Koha/Authorities.pm.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>