rhg: Remove `rhg.fallback-executable=hg` default configuration
When `rhg.on-unsupported` is configured to `fallback` and an unsupported
feature is encountered, the previous default was to look for an `hg`
executable in `$PATH`.
This default was fragile since it was easy to end up accidentally using
an older version of Mercurial installed system-wide,
when a local (perhaps patched) installation was intended.
Instead, it is now an error to have `rhg.on-unsupported=fallback`
without also configuring an explicit path or the fallback executable.
Differential Revision: https://phab.mercurial-scm.org/D10189
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}