After the dust settles from converting UnifiedComplete into UrlbarProviderPlaces (bug 1717509), we should clean up UrlbarProviderPlaces. Non-exhaustive list of tasks:

• UnifiedComplete currently has some code that performs a similar role as the muxer. Namely, deduping results in _getInsertIndexForMatch. We should move that functionality to the muxer and remove UnifiedComplete's system of "match keys" if possible.
• Remove now-unused code paths in UnifiedComplete. This includes unused methods that weren't caught by lint because they're private, like _addSearchEngineMatch.