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 .

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