.arcconfig

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

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}