Bug 15804: Use standard dialog style for confirmation of MARC subfield deletion

Bug 15804: Use standard dialog style for confirmation of MARC subfield deletion

When deleting a MARC framework subfield, the confirmation message is not
styled. This patch modifies the template to style the confirmation
message like similar ones.

To test, apply the patch and go to Administration -> MARC framworks.

 - Click 'MARC structure' for a framework you can edit.
 - Click 'subfields' for a tag you can edit.
 - Click 'Delete' for a subfield you can delete. You should see a
   confirmation dialog styled like other "alert" type dialogs.
 - Test both canceling and confirming deletion to confirm that both
   function correctly.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>