message: Remove unused a gn variable related to gtk
  This is not used anywhere.
  Cr-Commit-Position: refs/heads/master@{#33750}
diff: @@ -244,10 +244,6 @@ declare_args() {
  rtc_build_libevent = !build_with_mozilla
  }
  - # Build sources requiring GTK. NOTICE: This is not present in Chrome OS
  - # build environments, even if available for Chromium builds.
  - rtc_use_gtk = !build_with_chromium && !build_with_mozilla
  -
  # Excluded in Chromium since its prerequisites don't require Pulse Audio.
  rtc_include_pulse_audio = !build_with_chromium
  