Drop content_metadata table and indexer.
Depends on !191 (closed). Mutually exclusive with !445 (closed).
Motivation: the content_metadata table is currently used only as a cache for indexing revision/origin intrinsic metadata. But this cache is not properly handled, and not invalidated when indexers are updated. Fixing the issue would require extensive changes, and I don't think they are worth it; because content_metadata is a lot of complexity to handle, for very little performance benefit as a cache; and there is currently no use other than cache.
So I think we should remove it, and maybe add it back later if needed.
Migrated from D1838 (view on Phabricator)