Remove deprecated video_frame_i420 build target

Bug: chromium:1132299
Change-Id: Ie0207037b46d4c40d2d808bf5fcddf71f43e325b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187350
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32385}

api/video/BUILD.gn

@@ -74,13 +74,6 @@ if (is_android) {
   }
 }
 
-# Deprecated empty target. Use "video_frame" instead.
-rtc_source_set("video_frame_i420") {
-  visibility = [ "*" ]
-  sources = []
-  deps = [ ":video_frame" ]
-}
-
 rtc_library("video_frame_i010") {
   visibility = [ "*" ]
   sources = [