Bug 37152: Deleting patron categories doesn't warn about categories in use

You can't delete a patron category when it is in use, but because the
script which handles delete confirmation is expecting the wrong op,
cud-delete_confirm rather than delete_confirm, you don't get any warning,
just a failure to delete error message.

Test plan:
 1. Without the patch, Administration - Patron categories
 2. On the row for Staff, click Delete, Delete this category
 3. Note an error message telling you to check the logs for details
 4. Apply patch, restart_all
 5. With the patch, repeat steps 1-2, note that instead of a button for
    Delete this category you get a warning that it is in use
 6. In the list of categories, row for Board, click Delete, Delete this category
 7. Note that the (unused) Board category was deleted

Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit d913d5ab79)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

admin/categories.pl

@@ -168,7 +168,7 @@ elsif ( $op eq 'cud-add_validate' ) {
     $searchfield = q||;
     $op = 'list';
 }
-elsif ( $op eq 'cud-delete_confirm' ) {
+elsif ( $op eq 'delete_confirm' ) {
 
     my $count = Koha::Patrons->search({
         categorycode => $categorycode