memctx: simplify _manifest with new revlog nodeids This was originally written before we had modifiednodeid and addednodeid, so we had to get the parents of the context, the data from the function, and then hash that. This is much more simple now and helps refactor more code later.

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.