Unused LANGUAGE pragma Warning should suggest the redundant ones
Motivation
Currently (GHC 8.6.4), the warning triggered by unused pragmas only contains the full list of language extensions activated in a module ("Found ...") and the reduced set ("Why not ...").
Proposal
I think it would be valuable and informative to also include their set difference in the warning text (e.g. "Consider removing ..." or "Unused extensions: ..."). Perhaps this could be fit in the scope of #12543 .