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}

modules/video_coding/video_receiver_unittest.cc

@@ -57,7 +57,6 @@ class TestVideoReceiver : public ::testing::Test {
     // Register decoder.
     receiver_.RegisterExternalDecoder(&decoder_, kUnusedPayloadType);
     webrtc::test::CodecSettings(kVideoCodecVP8, &settings_);
-    settings_.plType = kUnusedPayloadType;
     EXPECT_EQ(
         0, receiver_.RegisterReceiveCodec(kUnusedPayloadType, &settings_, 1));