Remove redundant GHC extentions.
Remove redundant GHC extentions.

izgzhen/hadrian,bgamari/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,quchen/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian