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.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif