And remove unused imports and language pragmas.

I checked that the minimum Happy and Alex version requirements, as
listed in aclocal.m4, don't have to change. Before building ghc, I ran:
  - cabal install happy==1.19.4 --with-ghc=ghc-7.8.4
  - cabal install alex==3.1.0 --with-ghc=ghc-7.6.3

Differential Revision: https://phabricator.haskell.org/D1032