Prepare for deleting VideoCodec::plType

Deletes all webrtc usage of this member. Next step is to delete
any downstream references, and when that's done, the member can be
deleted.

Bug: None
Change-Id: I3f3a94a063dccf56468a1069653efd3809875b01
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/181201
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31911}

api/video_codecs/video_codec.h

@@ -103,6 +103,8 @@ class RTC_EXPORT VideoCodec {
 
   // Public variables. TODO(hta): Make them private with accessors.
   VideoCodecType codecType;
+  // TODO(nisse): Unused in webrtc, delete as soon as downstream projects are
+  // updated.
   unsigned char plType;
 
   // TODO(nisse): Change to int, for consistency.