cleanup: say goodbye to manifestv2 format This experiment was a bust: we'd hoped for smaller repository sizes, but things got larger. Google ended up rolling out tree manifests in a format that's compatible with the original manifest format, and I believe Facebook is doing the same. This code was never implemented as native speedups, so I'm pretty comfortable saying nobody is using the experimental feature. Let's rip it out. I noticed this code still kicking around because I was investigating a repo corruption issue for timeless. .. bc:: Support for the experimental manifestv2 format has been removed, as it was never completed and failed to meet expectations. Differential Revision: https://phab.mercurial-scm.org/D2393