Remove legacy video codec factories.

Removes the deprecated video codec factories and the related flag and
helper classes.

Bug: webrtc:7925
Change-Id: I0a6d1666ece9ad074fefc79b626ba241765e1b98
Reviewed-on: https://webrtc-review.googlesource.com/c/113940
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26245}

api/peer_connection_interface.h

@@ -120,11 +120,6 @@ class SSLIdentity;
 class Thread;
 }  // namespace rtc
 
-namespace cricket {
-class WebRtcVideoDecoderFactory;
-class WebRtcVideoEncoderFactory;
-}  // namespace cricket
-
 namespace webrtc {
 class AudioDeviceModule;
 class AudioMixer;