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.
#require docutils gettext
Error: the current ro localization has some rst defects exposed by
moving pager to core. These two warnings about references are expected
until the localization is corrected.
$ $TESTDIR/check-gendoc ro
checking for parse errors
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.