Delete unused function SetSessionError.

https://webrtc-codereview.appspot.com/47589004/ remove the use.

BUG=505316
Originally reviewed at https://codereview.webrtc.org/1296103002/
TBR=sergeyu@chromium.org

Review URL: https://codereview.webrtc.org/1299703002 .

Cr-Commit-Position: refs/heads/master@{#9719}
diff --git a/talk/session/media/channel.cc b/talk/session/media/channel.cc
index 8de8d37..5eec47c 100644
--- a/talk/session/media/channel.cc
+++ b/talk/session/media/channel.cc
@@ -60,11 +60,6 @@
 
 static const int kAgcMinus10db = -10;
 
-static void SetSessionError(BaseSession* session, BaseSession::Error error,
-                            const std::string& error_desc) {
-  session->SetError(error, error_desc);
-}
-
 static void SafeSetError(const std::string& message, std::string* error_desc) {
   if (error_desc) {
     *error_desc = message;