Remove self-registration for unaffiliated users

Currently Prosody is configured to allow membership registration in MUC rooms by unaffiliated users. Membership is controlled through the Django app, and should be read-only to Prosody.

Assuming this wasn't added for any particular reason, remove it.

See also https://git.happy-dev.fr/startinblox/applications/sib-app/issues/258#note_60134