mac/remote: remove make current observer when stopped

the observer is registered on every call of the start function. this
could lead to several registered observers for the same event and
several calls to the make current function, even though we only need it
once per event.

properly remove the observer on stop, so we only ever have one observer
registered.

osdep/macos/remote_command_center.swift

@@ -124,6 +124,12 @@ class RemoteCommandCenter: NSObject {
 
         infoCenter.nowPlayingInfo = nil
         infoCenter.playbackState = .unknown
+
+        NotificationCenter.default.removeObserver(
+            self,
+            name: NSApplication.willBecomeActiveNotification,
+            object: nil
+        )
     }
 
     @objc func makeCurrent(notification: NSNotification) {