Remove graph.mli
Remove graph.mli

Summary: `graph.ml` was removed in D5867496, but its `mli` file is still here to this day. I'm honestly surprised this even built.

Reviewed By: mvitousek

Differential Revision: D18511069

fbshipit-source-id: dd191408ee191baac4f9dea74853d9987fb4229f

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow