remove_ovl_mapping: string -> {Name:string,Thy:string} -> unit
A call to remove_ovl_mapping s {Name,Thy}, maps the Name-Thy record to a constant c, and removes the c-s pair from both maps.
As with other parsing functions, there is a sister function, temp_remove_ovl_mapping that does the same thing, but whose effect is not saved to a theory file.