ghc: revert "propagate 'gmp' and 'ncurses' to ensure users can link the 'haskeline' library, etc."

I thought that [1] could be fixed by ensuring that ncurses is available in the
environment (because ghc exports it as a propagateBuildInput), and indeed that
change fixed *some* build failures we've had before. However, the same error
still occurs with other packages, like hledger [2] and Agda [3]. Frankly, I
have no idea why those packages fail and others don't. But clearly the fix was
inadequate, so I'm reverting commit a8076c76.

[1] https://github.com/NixOS/nixpkgs/issues/5616
[2] http://hydra.cryp.to/build/372451/nixlog/1/raw
[2] http://hydra.cryp.to/build/373161/nixlog/1/raw