remotefilelog: remove support for fetching packs We've never used this at Google, and it's strictly an optimization that indygreg and I are confident we can handle more elegantly later. Let's rip it out to keep the surface area of the imported remotefilelog small. Some tests have changes because the tests for regular packs and repacking were mixed with tests of fetching packs. I'm pretty sure I've done the right thing here. Differential Revision: https://phab.mercurial-scm.org/D5122