LanguageClient: use separate variable to save shutdown state

The LanguageClientManager might be already gone.

Change-Id: Ia674a8c76e0c1bb39e1503be9cffc20fd69120bb
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientmanager.h

@@ -100,7 +100,6 @@ private:
 
     QList<Client *> reachableClients();
 
-    bool m_shuttingDown = false;
     QList<Client *> m_clients;
     QList<BaseSettings *>  m_currentSettings; // owned
     QMap<QString, QList<Client *>> m_clientsForSetting;