Disable ghc-mod as the checker used in Syntastic, in favor of hdevtools

Reasons:
1. hdevtools serves almost the same functionality as ghc-mod
2. ghc-mod is much slower than hdevtools, which makes the Vim GUI hang
3. ghc-mod outputs the not-very-nice <Nul> chars instead of newlines

But we still keep the corresponding Vim plugin ghcmod-vim around in case
one wishes to invoke the purportedly-more-powerful ghc-mod inside Vim.