audio_manager: Remove unused sessions_started member variable

This isn't used, so it can be removed.

src/audio_core/audio_manager.h

@@ -76,8 +76,6 @@ private:
      */
     void ThreadFunc();
 
-    /// Have sessions started playing?
-    bool sessions_started{};
     /// Is the main thread running?
     std::atomic<bool> running{};
     /// Unused