Drop "testing" cabal flag

Since 8863761d66 (early 2014), the test
suite is a proper cabal test-suite and needs the "xmonad" library to be
built, and thus the flag serve no purpose.

Related: https://github.com/xmonad/xmonad/issues/283

xmonad.cabal

@@ -51,11 +51,6 @@ source-repository head
   type:     git
   location: https://github.com/xmonad/xmonad
 
-flag testing
-  default:     False
-  manual:      True
-  description: Testing mode, only build minimal components
-
 flag generatemanpage
   default:     False
   manual:      True
@@ -89,9 +84,6 @@ library
   if impl(ghc <= 8.4.4)
      ghc-options: -Werror=unused-imports
 
-  if flag(testing)
-    buildable: False
-
 executable xmonad
   main-is:       Main.hs
   build-depends: base, X11, mtl, unix, xmonad