Remove old Authentication code that we don't use or need.

remoting/proto/control.proto

Index: remoting/proto/control.proto
diff --git a/remoting/proto/control.proto b/remoting/proto/control.proto
index 1f75bcb0500729fa1d45ed3305ec07efa0dc7dea..a86ea5e94504771c633755df6756005c21ee8b5d 100644
--- a/remoting/proto/control.proto
+++ b/remoting/proto/control.proto
@@ -10,4 +10,15 @@ option optimize_for = LITE_RUNTIME;
 
 package remoting.protocol;
 
-// Add control messages here. Currently we don't have any.
+// BeginSessionDeprecated is sent from host to client when the session
+// starts. Legacy clients expect to receive this message at the
+// beginning of each session. Current clients ignore it.
+//
+// TODO(sergeyu): Remove it once all clients are upgraded to the new
+// version.
+message LocalLoginStatusDeprecated {
+  optional bool success = 1;
+}
+message BeginSessionDeprecated {
+  optional LocalLoginStatusDeprecated login_status = 1;
+}