Bug 33697: Remove RecordedBooks (rbdigital) integration

Bug 33697: Remove RecordedBooks (rbdigital) integration

RecordedBooks search API integration is now obsolete following
rbdigital's incorporation into OverDrive. Associated code should be
removed.

https://company.overdrive.com/2020/06/23/overdrive-to-acquire-rbdigital-from-rbmedia/

Test plan:
use git grep extensively and confirm that this patch removes all
occurrences of this feature.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>