Removes dependency on ghc-syb that, according to Hackage, doesn't exist.

After update of the ghc-syb-utils (from 0.2.1.1 to 0.2.1.2) ghc-mod fails to build under ghc-7.6.3. The reason, I think, is that the new version of ghc-syb-utils imposes a dependency on ghc-syb that according to hackage doesn't exist.