ui: accessibility: remove -Wunused-function for Atk

Newer version of GLib define *get_instance_private symbols in
the G_DEFINE_TYPE macros. These symbols might be unused in cases
where a GObject class doesn't access its instances private data,
triggering -Wunused-function warnings.

BUG=504375
TEST=compile with gn on linux with GLib >= 2.37

Review URL: https://codereview.chromium.org/1216153005

Cr-Original-Commit-Position: refs/heads/master@{#338001}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 991e92d77c0b9f46c6a5d3f6facd12861c9e1828

config/linux/BUILD.gn

@@ -35,25 +35,6 @@ config("sdk") {
   }
 }
 
-pkg_config("atk") {
-  packages = [ "atk" ]
-  atk_lib_dir = exec_script(pkg_config_script,
-                            [
-                              "--libdir",
-                              "atk",
-                            ],
-                            "string")
-  defines = [ "ATK_LIB_DIR=\"$atk_lib_dir\"" ]
-}
-
-# gn orders flags on a target before flags from configs. The default config
-# adds -Wall, and these flags have to be after -Wall -- so they need to come
-# from a config and can't be on the target directly.
-config("atk_warnings") {
-  # glib uses the pre-c++11 typedef-as-static_assert hack.
-  cflags = [ "-Wno-unused-local-typedef" ]
-}
-
 config("fontconfig") {
   libs = [ "fontconfig" ]
 }