Remove SpdySession::GetSSLClientSocket

After removing SpdyStream::GetSSLCertRequestInfo, in
https://codereview.chromium.org/1128873008, this method
looks like unused.

BUG=

Review URL: https://codereview.chromium.org/1346133004

Cr-Commit-Position: refs/heads/master@{#349697}
2 files changed