Removed support for Generics.

There are two good reasons to remove all support for Generic Haskell from haskell-src-exts:

1. As far as I know, Generic Haskell is dead. The last version of the official compiler was released over five years ago (April 2008) and fails to build from source. The graduate course "Generic Programming" at Utrecht University (which hosts the GH compiler) only mentions Generic Haskell as a historical artifact.
2. Generic Haskell support at type-level is completely broken. The only support seems to be of `{| |}` in patterns, but `haskell-src-exts` chokes on `{[ ]}` in type signatures.

If I `-XGenerics` is distinct from Generic Haskell, it's a good idea to remove it anyway: [the option was removed from GHC two years ago](http://ghc.haskell.org/trac/ghc/changeset/b8f32576379b4e094f0a64fcca8ef3b13782975a/ghc) (July 2011).