Bug 41008: Fix bulkmarimport.pl -a -d

After bug 35659 that added the OAI harvester to Koha the -d switch
for deleting authorities before importing with bulkmarcimport.pl
was broken and resulted in a FK error.

Switching from truncate to DELETE FROM like we already did for items,
biblio etc. fixes the issue.

To test:
- Make sure you have some authorities in your database
- Export some into a file for importing
- Run bulkmarcimport -a -d --file <filename>
- Verify you get the a DB FK constraint error.
- Apply patch
- Repeat steps, verify the script now works as expected again
  * Existing authorities are deleted
  * File is imported

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Jan Kissig <bibliothek@th-wildau.de>

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 59d07337de)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>

misc/migration_tools/bulkmarcimport.pl

@@ -236,7 +236,7 @@ if ($delete) {
         $dbh->do("ALTER TABLE items AUTO_INCREMENT = 1");
     } else {
         print "Deleting authorities\n";
-        $dbh->do("truncate auth_header");
+        $dbh->do("DELETE FROM auth_header");
     }
     $dbh->do("truncate zebraqueue");
 }