Bug 35753: Remove Checkbox() function

Bug 35753: Remove Checkbox() function

To test:
1. APPLY PATCH and clear browser cache.
2. Have multiple additional content items ( news/HTML customization/Pages )
3. From one of the pages ( news/HTML customization/Pages ) click the 'Delete selected' button. Since no items are checked you should see the 'Please select content to delete' alert.
4. Check some items and click 'Delete selected' they should be correctly deleted.

Note: There is no change in behavior from this patch.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>