Delete unused function SetSessionError.
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9719}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: dbe5bd9ad58c9d53289ff678b2db54433ac63a07

sippet/talk,sippet/talk,sippet/talk,sippet/talk,sippet/talk