Clean up Faiss, purge `jina_commons`

Faiss should be cleaned up - the whole rationale behind having the complicated dumping setup is that `faiss.write_index` does not save indexed vectors - which is not true (was it even checked?). So we can simplify FaissSearcher by cutting out significant portions of current code, and getting rid of `jina_commons` dependency, which does not serve any purpose anymore