help: hide default value for default-off flags
If we no longer show the "[no-]" for default-off flags, it also seems
unnecessary to show the "default: off" for them, since that's quite
clearly the default. It's extra confusing for action flags like `hg
bookmarks --delete`.
Differential Revision: https://phab.mercurial-scm.org/D5455
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}