Assistant: Remove stale documentation registrations at startup

A qch file holds exactly one namespace. If a file was regenerated for
another version (e.g. after a version bump) and was not registered
again, the collection still holds the old namespace for it. Compare
the registered namespaces with the ones stored in the files at startup
and unregister the stale ones. Files that can't be read are left
registered.

Pick-to: 6.12
Change-Id: Id1fdbe8c65f2351f0a425a2cf730f8fddc6056c2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>