Remove deprecated DirectTransport constructors

Remove deprecated DirectTransport constructors, now that they're no longer used in internal projects.

BUG=webrtc:8125
TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/3006513002
Cr-Original-Commit-Position: refs/heads/master@{#19496}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0860bee4026639f22acd53ea144731103f578448
2 files changed