Revise dependencies a bit

utf8-string isn't needed since d9e3ebf531
(late 2014).

Most of the executable xmonad deps are unnecessary since
307b82a53d (early 2015).

Switch from data-default to data-default-class to avoid bringing in deps
and instances we don't need.

src/XMonad/Config.hs

@@ -39,7 +39,7 @@ import XMonad.Operations
 import XMonad.ManageHook
 import qualified XMonad.StackSet as W
 import Data.Bits ((.|.))
-import Data.Default
+import Data.Default.Class
 import Data.Monoid
 import qualified Data.Map as M
 import System.Exit

src/XMonad/Core.hs

@@ -43,7 +43,7 @@ import Control.Monad.State
 import Control.Monad.Reader
 import Data.Semigroup
 import Data.Traversable (for)
-import Data.Default
+import Data.Default.Class
 import System.FilePath
 import System.IO
 import System.Info

xmonad.cabal

@@ -71,7 +71,7 @@ library
   build-depends:   base                  >= 4.9 && < 5
                  , X11                   >= 1.10 && < 1.11
                  , containers
-                 , data-default
+                 , data-default-class
                  , directory
                  , filepath
                  , mtl
@@ -79,7 +79,6 @@ library
                  , setlocale
                  , transformers          >= 0.3
                  , unix
-                 , utf8-string           >= 0.3 && < 1.1
   ghc-options:     -funbox-strict-fields -Wall -Wno-unused-do-bind
 
   -- Keep this in sync with the oldest version in 'tested-with'
@@ -91,7 +90,7 @@ library
 
 executable xmonad
   main-is:       Main.hs
-  build-depends: base, X11, mtl, unix, xmonad
+  build-depends: base, xmonad
   ghc-options:   -Wall -Wno-unused-do-bind
 
   -- Keep this in sync with the oldest version in 'tested-with'