remotefilelog: rip out lz4 support Some methods are left teased out so it's easier to build a migration extension from v1 packs to v2. I also anticipate those methods will be of use if we get around to adding configurable compression engines before we end up jettisoning packs. I think it would make sense to register one-shot `compress` and `decompress` methods on our compression engines in util.py, but indygreg mentioned that might not be straightforward. Differential Revision: https://phab.mercurial-scm.org/D4944