Remove duplicate entry in webrtc .gn file exec_script_whitelist

In the file .gn exec_script_whitelist has 2 copies of
  "//webrtc/modules/video_render/BUILD.gn",

This CL removes the duplicate

BUG=webrtc:6527
TEST=try bots
R=kjellander@webrtc.org

Review-Url: https://codereview.webrtc.org/2414953002
Cr-Commit-Position: refs/heads/master@{#14621}
diff --git a/.gn b/.gn
index db61a47..7a98fad 100644
--- a/.gn
+++ b/.gn
@@ -65,5 +65,4 @@
   "//third_party/openh264/BUILD.gn",
   "//third_party/opus/BUILD.gn",
   "//webrtc/modules/video_render/BUILD.gn",
-  "//webrtc/modules/video_render/BUILD.gn",
 ]