sync: Remove dead code related to ClearUserData

components/sync_driver/sync_frontend.h

Index: components/sync_driver/sync_frontend.h
diff --git a/components/sync_driver/sync_frontend.h b/components/sync_driver/sync_frontend.h
index 2b30df15736b06aaf9563fef2887b0aaa1b7c357..f7ed853d50bd3a167651db034454dd66be5d5936 100644
--- a/components/sync_driver/sync_frontend.h
+++ b/components/sync_driver/sync_frontend.h
@@ -57,10 +57,6 @@ class SyncFrontend {
   virtual void OnConnectionStatusChange(
       syncer::ConnectionStatus status) = 0;
 
-  // We are no longer permitted to communicate with the server. Sync should
-  // be disabled and state cleaned up at once.
-  virtual void OnStopSyncingPermanently() = 0;
-
   // The syncer requires a passphrase to decrypt sensitive updates. This is
   // called when the first sensitive data type is setup by the user and anytime
   // the passphrase is changed by another synced client. |reason| denotes why