Bug 29086: Simplify querying active article requests

Bug 29086: Simplify querying active article requests

This patch makes a trivial replacement of a query for a higher level
method that does the same query.

To test:
1. Enable the ArticleRequests feature
2. Place some article requests on known biblios
3. Have some article requests be active on others finished (either
   completed or cancelled)
4. Go to the detail page for those records
=> SUCCESS: The active article requests count is correct!
5. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>