Deprecate swh.core's postgresql_fact

This factory function is not really needed any more. It was mostly used to speed postgresql-based tests (using custom setup/teardown methods doing table truncation instead of full db drop and recreate).

Recent(ish) versions of pytest_postgresql do provide a db creation process speedup using a preconfigured and setup template db to create the database used in each test.

With the work done in f8a07dfa (an following revisions), we do not need this custom setup/teardown any more, so it should be deprecated and its usage adapted to stock pytest_postgresql factory function in every dependency still using it.

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Migrated from T4285 (view on Phabricator)