datastore: Revert use of materialized views (v14.4)

Revert the introduction of materialized views for valid_primaries. As it turns out, the changes cause incompatibilities with Postgres 11, which is still actively in use. Furthermore, the performance issues we have seen have not been fully fixed with this change, and we do not yet fully understand the root cause for this.

Fixes gitlab-com/gl-infra/production#5945 (closed)