Bug 26638: Remove unused system preference ArticleRequestsMandatoryFieldsItemsOnly

Bug 26638: Remove unused system preference ArticleRequestsMandatoryFieldsItemsOnly

The preference was probably added because of a typo, as there is a
used system preference ArticleRequestsMandatoryFieldsItemOnly
(Item vs Items).

This patch fixes the situation:

- syspref.sql had the pref name with the typo, this is fixed.
- A database update removes the unused variable and makes sure
  the used one is added.

To test:
- Look at the local use system preferences, verify the false pref is there
- Look at the other ArticleRequests* preferences, verify the other without "s"
  is there
- Apply patch and run database update
- Verify the local use tab one is gone now

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>