Remove OnOptInChanged callback.

chrome/browser/chromeos/arc/arc_auth_service.h

Index: chrome/browser/chromeos/arc/arc_auth_service.h
diff --git a/chrome/browser/chromeos/arc/arc_auth_service.h b/chrome/browser/chromeos/arc/arc_auth_service.h
index 28074f8f4e43e2267b1dffb755317129d0700c63..0f789784cf53443bdf81be08c01b961facdbccdf 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.h
+++ b/chrome/browser/chromeos/arc/arc_auth_service.h
@@ -77,9 +77,6 @@ class ArcAuthService : public ArcService,
    public:
     virtual ~Observer() = default;
 
-    // Called whenever Opt-In state of the ARC has been changed.
-    virtual void OnOptInChanged(State state) {}
-
     // Called to notify that ARC bridge is shut down.
     virtual void OnShutdownBridge() {}