retire support for Generics

Not only has `Generics` been [deprecated in GHC for over two years](http://ghc.haskell.org/trac/ghc/changeset/b8f32576379b4e094f0a64fcca8ef3b13782975a/ghc) (July 2011), but I can't find any documentation or enough example code that references it. For me, that would be enough reason to scrap the feature.