remove redundant imports

Clean build under ghc 8.8.3, which seems to do better at finding cases
where two imports both provide the same symbol, and warns about one of
them.

This commit was sponsored by Ilya Shlyakhter on Patreon.

Utility/SimpleProtocol.hs

@@ -25,7 +25,6 @@ module Utility.SimpleProtocol (
 
 import Data.Char
 import GHC.IO.Handle
-import System.Exit (ExitCode(..))
 
 import Common