Remove unused a gn variable related to gtk

This is not used anywhere.

Bug: none
Change-Id: I620739aa7e73f6b82c67dd89972a01a37f67c149
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215380
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33750}
diff --git a/webrtc.gni b/webrtc.gni
index 249ae62..ac6c325 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -244,10 +244,6 @@
     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