Removed undefined method from webrtcsession.h.

It was accidentally added in https://codereview.webrtc.org/2583883002/
(added in one patch set, removed in another but forgot about the header
declaration).

BUG=webrtc:6875, chromium:627816

Review-Url: https://codereview.webrtc.org/2583123003
Cr-Original-Commit-Position: refs/heads/master@{#15685}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7472dc3b9444cb05b17023b87dd3e8016773f359
1 file changed