Bug 18070: [Follow-up] What should DelAuthority return?

Bug 18070: [Follow-up] What should DelAuthority return?

Script tools/batch_delete_records.pl only checks the return value of
DelAuthority. The return value depends on DBI rows, which is not always
reliable. It may return -1 when it 'does not know'.

Testing $@ in tools/batch_delete_records.pl should actually be enough.
The return value was discarded in C4/ImportBatch.pm. Removing an unused
variable.

Test plan:
[1] Pick an authority record with a few linked biblios.
    Delete this authority record via tools/batch_delete_records.pl.
    Check if the linked biblio records are cleaned up.
[2] Bonus: Make a typo in the SQL statement of DelAuthority. Check
    if batch_delete_records shows you the error message.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>