Bug 37091: Correct op parameter in systempreferences.pl

Bug 37091: Correct op parameter in systempreferences.pl

To test:
1. Go to Administration > System Preferences > Local Use
2. Attempt to delete a local system preference
--> The confirmation page displays a blank name and value
3. Click "Yes, delete"
--> The system preference is still there
4. Apply patch, restart_all, and reload page
5. Attempt to delete the system preference again
--> The confirmation page displays correct information
6. Click "Yes, delete"
--> The system preference is deleted successfully

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>