We didn't really want it; it was only necessary because we wanted to
use rtc::Optional<SdpAudioFormat>, and Optional used to require the
contained type to be default constructable. But as of May 9th
(https://codereview.webrtc.org/1896833004), it no longer does.
Review-Url: https://codereview.webrtc.org/2066233002
Cr-Commit-Position: refs/heads/master@{#13211}
44bf02fba2