Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1480

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1242

This bridge doesn't support SQLite anyway, so it's not necessary
to carry around configuration fields and code for migration from SQLite
to Postgres. There's nothing to migrate.

This configuration is supposed to be kept clean and not reference variables defined in other roles.

`group_vars/matrix_servers` redefines these to hook our various roles together.