Revert "Implement read-only codecPayloadType in RtpParameters"

This reverts commit 806e06d1366b58878ced05cdd8d1d56394982fe6.

Reason for revert: Breaks WebRTC roll to Chromium. https://chromium-review.googlesource.com/c/chromium/src/+/1375538

02:52:35.346 7748   [6936:11248:1213/025234.206:ERROR:mediaengine.cc(80)] Attempted to set RtpParameters with modified codecPayloadType (INVALID_MODIFICATION)

Original change's description:
> Implement read-only codecPayloadType in RtpParameters
> 
> Bug: webrtc:7580
> Change-Id: I6d901afa97262b6c6d9fe6c7366df465ec77bfb3
> Reviewed-on: https://webrtc-review.googlesource.com/c/113944
> Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
> Reviewed-by: Seth Hampson <shampson@webrtc.org>
> Reviewed-by: Anders Carlsson <andersc@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Commit-Queue: Florent Castelli <orphis@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#25993}

TBR=steveanton@webrtc.org,sakal@webrtc.org,andersc@webrtc.org,shampson@webrtc.org,orphis@webrtc.org

Change-Id: I157f9a79ae7133395431891e15e2c053559d359b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:7580
Reviewed-on: https://webrtc-review.googlesource.com/c/114300
Reviewed-by: Henrik Grunell <henrikg@webrtc.org>
Commit-Queue: Henrik Grunell <henrikg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26000}
diff --git a/api/rtpparameters.h b/api/rtpparameters.h
index f4b5198..47df22e 100644
--- a/api/rtpparameters.h
+++ b/api/rtpparameters.h
@@ -377,7 +377,12 @@
   // unset SSRC acts as a "wildcard" SSRC.
   absl::optional<uint32_t> ssrc;
 
-  // Read-only parameter indicating the payload type of the codec being used.
+  // Can be used to reference a codec in the |codecs| member of the
+  // RtpParameters that contains this RtpEncodingParameters. If unset, the
+  // implementation will choose the first possible codec (if a sender), or
+  // prepare to receive any codec (for a receiver).
+  // TODO(deadbeef): Not implemented. Implementation of RtpSender will always
+  // choose the first codec from the list.
   absl::optional<int> codec_payload_type;
 
   // Specifies the FEC mechanism, if set.