Remove SdpAudioFormat's default constructor

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-Original-Commit-Position: refs/heads/master@{#13211}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 44bf02fba2d0d6c2c1b08ea09e2a0cd2e8b93846
2 files changed