Revert "Add image_diff from the chromium repo."

sky/tools/imagediff/BUILD.gn

Index: sky/tools/imagediff/BUILD.gn
diff --git a/sky/tools/imagediff/BUILD.gn b/sky/tools/imagediff/BUILD.gn
deleted file mode 100644
index 887f4123f333eda2350f7b4158f050ce2ab5d00d..0000000000000000000000000000000000000000
--- a/sky/tools/imagediff/BUILD.gn
+++ /dev/null
@@ -1,20 +0,0 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-executable("imagediff") {
-  output_name = "image_diff"  # Different than dir nam for historical reasons.
-  sources = [
-    "image_diff.cc",
-    "image_diff_png.h",
-    "image_diff_png.cc",
-  ]
-
-  configs += [ "//build/config/compiler:wexit_time_destructors" ]
-
-  deps = [
-    "//base",
-    "//third_party/libpng",
-    "//third_party/zlib",
-  ]
-}