[Clean-up] Remove some dead code for handling Webstore sign-in.

components/signin/core/browser/signin_manager.h

Index: components/signin/core/browser/signin_manager.h
diff --git a/components/signin/core/browser/signin_manager.h b/components/signin/core/browser/signin_manager.h
index 5868e0d907b23198981aa780045d2d0e5c074f7a..f03beb7522442bd76d183723356e2dbf798ecb1e 100644
--- a/components/signin/core/browser/signin_manager.h
+++ b/components/signin/core/browser/signin_manager.h
@@ -130,10 +130,8 @@ class SigninManager : public SigninManagerBase {
   bool IsSignoutProhibited() const;
 
   // Add or remove observers for the merge session notification.
-  virtual void AddMergeSessionObserver(
-      MergeSessionHelper::Observer* observer);
-  virtual void RemoveMergeSessionObserver(
-      MergeSessionHelper::Observer* observer);
+  void AddMergeSessionObserver(MergeSessionHelper::Observer* observer);
+  void RemoveMergeSessionObserver(MergeSessionHelper::Observer* observer);
 
  protected:
   // Flag saying whether signing out is allowed.