Remove unused imports.

Due to a bug in ghc, some unused imports do not yield warnings.
This commit will remove such unused imports in preparation for
the ghc bug fix (see https://ghc.haskell.org/trac/ghc/ticket/13064).

Cabal/Distribution/Compat/Prelude.hs

@@ -4,9 +4,11 @@
 {-# LANGUAGE FlexibleContexts #-}
 
 #ifdef MIN_VERSION_base
+#define MINVER_base_411 MIN_VERSION_base(4,11,0)
 #define MINVER_base_48 MIN_VERSION_base(4,8,0)
 #define MINVER_base_47 MIN_VERSION_base(4,7,0)
 #else
+#define MINVER_base_411 (__GLASGOW_HASKELL__ >= 804)
 #define MINVER_base_48 (__GLASGOW_HASKELL__ >= 710)
 #define MINVER_base_47 (__GLASGOW_HASKELL__ >= 708)
 #endif
@@ -90,7 +92,6 @@ module Distribution.Compat.Prelude (
     -- * Text.PrettyPrint
     (<<>>),
     ) where
-
 -- We also could hide few partial function
 import Prelude                       as BasePrelude hiding
   ( IO, mapM, mapM_, sequence, null, length, foldr, any, all
@@ -117,7 +118,12 @@ import Control.DeepSeq               (NFData (..))
 import Data.Data                     (Data)
 import Data.Typeable                 (Typeable)
 import Distribution.Compat.Binary    (Binary (..))
-import Distribution.Compat.Semigroup (Semigroup (..), gmappend, gmempty)
+#if !MINVER_base_411 && MINVER_base_48
+import Distribution.Compat.Semigroup (Semigroup (..))
+#endif
+#if MINVER_base_48
+import Distribution.Compat.Semigroup (gmappend, gmempty)
+#endif
 import GHC.Generics                  (Generic, Rep(..),
                                       V1, U1(U1), K1(unK1), M1(unM1),
                                       (:*:)((:*:)), (:+:)(L1,R1))