context: remove unused overlayfilectx (API)
It seems that this was maybe used in an extension but at this point
nothing in lfs, hg-experimental, or any other cursory repo looked at has
a reference to this class; so, for now, let's just remove it.
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
}