[repo] Drop _get_session method
it's not actually necessary but holds underlying _sessions dict handling.
Drop backward compat relying on it (which IMO should not be a problem).
Drop import of unused QueryError along the way.
[test] Drop call to CubicWebConfiguration.load_cwctl_plugins in unittest_cwctl.py
This is no longer need now that we call load_available_configurations() in the
"list" command (see 5e7282bdf140).