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-Commit-Position: refs/heads/master@{#15685}

webrtc/api/webrtcsession.h

@@ -208,7 +208,6 @@ class WebRtcSession :
   virtual cricket::DataChannel* data_channel() {
     return data_channel_.get();
   }
-  std::unique_ptr<ChannelNamePairs> GetChannelNamePairs();
 
   cricket::BaseChannel* GetChannel(const std::string& content_name);