Remove LANGUAGE pragrams implied by Haskell2010

Haskell2010 implies (at least) EmptyDataDecls, ForeignFunctionInterface,
PatternGuards, DoAndIfThenElse, and RelaxedPolyRec.

This is a follow-up to dd92e2179e3171a0630834b773c08d416101980d

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>