Remove unimplemented (facebook#54585)

Summary:

# Changelog: [Internal]

There is no implementation for this method and it is unused. This is actually part of `HostAgent`.

Differential Revision: D87220192

Author: hoxyq

packages/react-native/ReactCommon/jsinspector-modern/HostTarget.h

@@ -326,11 +326,6 @@ class JSINSPECTOR_EXPORT HostTarget : public EnableExecutorFromThis<HostTarget>
    */
   void emitTraceRecordingForFirstFuseboxClient(tracing::TraceRecordingState traceRecording) const;
 
-  /**
-   * Emits a system state changed event to all active sessions.
-   */
-  void emitSystemStateChanged(bool isSingleHost) const;
-
  private:
   /**
    * Constructs a new HostTarget.