Centralize a PluginManager::isShuttingDown() status

Instead of keeping track of this in plugins individually.

Change-Id: Ia2650f0f647d4a63d2010cef688aa56f6020c338
Reviewed-by: hjk <hjk@qt.io>

src/plugins/languageclient/languageclientmanager.h

@@ -48,7 +48,6 @@ public:
     static void deleteClient(Client *client);
 
     static void shutdown();
-    static bool isShuttingDown();
 
     static LanguageClientManager *instance();