configuration-ghc-7.8.x.nix: remove amazonka-related overrides

These overrides allow us to build amazonka packages, but we have no way to
actually use those build results within Nix, because the replaced GHC core
libraries collide with the real ones when we try to compose a ghc-with-packages
environment.

Closes https://github.com/NixOS/nixpkgs/issues/6584.