Comments and tiny refactor

* Added Note [Quantified varaibles in partial type signatures]
  in TcRnTypes

* Kill dVarSetElemsWellScoped; it was only called in
  one function, quantifyTyVars.  I inlined it because it
  was only scopedSort . dVarSetElems

* Kill Type.tyCoVarsOfBindersWellScoped, never called.