core/telemetry_session: Only create the backend when we really need it

The backend is not used until we decide to submit the testcase/telemetry, and creating it early prevents users from updating the credentials properly while the games are running.

src/core/telemetry_session.h

@@ -39,7 +39,6 @@ public:
 
 private:
     Telemetry::FieldCollection field_collection; ///< Tracks all added fields for the session
-    std::unique_ptr<Telemetry::VisitorInterface> backend; ///< Backend interface that logs fields
 };
 
 /**