Suppress output of hlint

As you know, hlint >= 1.8.58 doesn't support `--quiet`.
Since 1ae10600e7c81b9d6cce69249ccd95c1c0cbac03, `ghc-mod lint` outputs each suggestion twice (from hlint with pretty-printed format and from ghc-mod with easy-to-parse format).

This pull-request suppresses output of hlint to restore the previous behavior.