Remove the `createDecoder(String)` overload

This method has been deprecated since 2018-07:
https://webrtc-review.googlesource.com/c/src/+/88368/

It is never called by WebRTC itself.

Custom `VideoDecoderFactory` implementations overriding this method can
switch to the overload accepting a `VideoCodecInfo` object.

This is also adding a `toString()` implementation to `VideoCodecInfo`,
to make logging of the value more useful.

Bug: webrtc:7925
Change-Id: I70ec07a0cd4ffa07d165c9851e393439fcc5870b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/221960
Commit-Queue: Xavier Lepaul‎ <xalep@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34302}