[sources] Drop source support_entity / support_relation source API
this was used in the pre-datafeed area, it's not worth it anymore.
Its only valid usage was in authentication to detect if the source was
supporting CWUser, hence we now call it systematically and catch
NotImplementedError.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*