Removing $rtc_libyuv_dir and removing useless dependencies on libyuv.

This CL removes the following GN variables: rtc_build_libyuv,
rtc_libyuv_dir (as requested in webrtc:7906).
It also removes some unneeded dependencies on //third_party/libyuv.

WebRTC targets were using public_deps to depend on //third_party/libyuv
and this created a build graph where targets that were depending on
//third_party/libyuv were not declaring the dependency to GN because
they were somehow getting it from another target that was exposing
//third_party/libyuv header files even if it wasn't directly depending
on it.

Bug: webrtc:8605, webrtc:7906
Change-Id: If71f7988fd80421dc2ad887cf94c2ac66366c3fb
Reviewed-on: https://webrtc-review.googlesource.com/32201
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21275}
diff --git a/common_video/BUILD.gn b/common_video/BUILD.gn
index 6c14100..07733db 100644
--- a/common_video/BUILD.gn
+++ b/common_video/BUILD.gn
@@ -60,7 +60,6 @@
   }
 
   deps = [
-    "$rtc_libyuv_dir",
     "..:webrtc_common",
     "../api:optional",
     "../api:video_frame_api_i420",
@@ -113,7 +112,6 @@
 
     deps = [
       ":common_video",
-      "$rtc_libyuv_dir",
       "../api:video_frame_api_i420",
       "../modules/video_capture:video_capture",
       "../rtc_base:rtc_base",
@@ -123,6 +121,7 @@
       "../test:video_test_common",
       "//testing/gmock",
       "//testing/gtest",
+      "//third_party/libyuv",
     ]
 
     data = common_video_resources