Clean up GTK3 build flags.

build/config/ui.gni

 # 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.
 
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # These flags are effectively global. Your feature flag should go near the
 # code it controls. Most of these items are here now because they control
@@ skipping 22 matching lines @@
   # of a replacement for GDI or GTK.
   use_aura = is_win || is_linux
 
   # True means the UI is built using the "views" framework.
   toolkit_views =
       (is_mac || is_win || is_chromeos || use_aura) && !is_chromecast
 
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
 
-  # Whether we should use GTKv3 instead of GTKv2.
-  use_gtk3 = false
-
   # Whether we should use glib, a low level C utility library.
   use_glib = is_linux
 
   # Indicates if Wayland display server support is enabled.
   enable_wayland_server = is_chromeos
 
   # Enable experimental vulkan backend.
   enable_vulkan = false
 }
 
@@ skipping 21 matching lines @@
   use_pango = false
 }
 
 # Whether to use atk, the Accessibility ToolKit library
 use_atk = is_desktop_linux && use_x11
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # See comment at the top.