Make CodecType conversion functions non-optional.

We can't handle no value here anyway and end up setting a default
at each call site. The defaults aren't even the same in each place.

BUG=None

Review-Url: https://codereview.webrtc.org/2998293002
Cr-Commit-Position: refs/heads/master@{#19485}
diff --git a/webrtc/modules/video_coding/video_codec_initializer.cc b/webrtc/modules/video_coding/video_codec_initializer.cc
index df8f136..d274bf1 100644
--- a/webrtc/modules/video_coding/video_codec_initializer.cc
+++ b/webrtc/modules/video_coding/video_codec_initializer.cc
@@ -104,8 +104,7 @@
 
   VideoCodec video_codec;
   memset(&video_codec, 0, sizeof(video_codec));
-  video_codec.codecType = PayloadNameToCodecType(payload_name)
-                              .value_or(VideoCodecType::kVideoCodecGeneric);
+  video_codec.codecType = PayloadStringToCodecType(payload_name);
 
   switch (config.content_type) {
     case VideoEncoderConfig::ContentType::kRealtimeVideo: