sync: Remove dead code related to ClearUserData

sync/internal_api/public/sync_manager.h

Index: sync/internal_api/public/sync_manager.h
diff --git a/sync/internal_api/public/sync_manager.h b/sync/internal_api/public/sync_manager.h
index de8ee792598a3169af30c5312235b9ef31bedf7d..54e12b64cd1532b334f7ceb6fbcfd52140004247 100644
--- a/sync/internal_api/public/sync_manager.h
+++ b/sync/internal_api/public/sync_manager.h
@@ -219,12 +219,6 @@ class SYNC_EXPORT SyncManager : public syncer::InvalidationHandler {
         bool success,
         ModelTypeSet restored_types) = 0;
 
-    // We are no longer permitted to communicate with the server. Sync should
-    // be disabled and state cleaned up at once.  This can happen for a number
-    // of reasons, e.g. swapping from a test instance to production, or a
-    // global stop syncing operation has wiped the store.
-    virtual void OnStopSyncingPermanently() = 0;
-
     virtual void OnActionableError(
         const SyncProtocolError& sync_protocol_error) = 0;