No more once_cell

Fix system audio check that didn't work because the API returns ()

src/session_notification.rs

@@ -23,9 +23,3 @@ impl IAudioSessionNotification_Impl for SessionNotification {
         Ok(())
     }
 }
-
-impl Drop for SessionNotification {
-    fn drop(&mut self) {
-        println!("SN drop");
-    }
-}