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/qmljseditor/qmljseditorplugin.cpp

@@ -210,11 +210,6 @@ void QmlJSEditorPlugin::extensionsInitialized()
     QmllsSettingsManager::instance()->setupAutoupdate();
 }
 
-ExtensionSystem::IPlugin::ShutdownFlag QmlJSEditorPlugin::aboutToShutdown()
-{
-    return IPlugin::aboutToShutdown();
-}
-
 Utils::JsonSchemaManager *QmlJSEditorPlugin::jsonManager()
 {
     return &m_instance->d->m_jsonManager;

src/plugins/qmljseditor/qmljseditorplugin.h

@@ -29,7 +29,6 @@ public:
 private:
     void initialize() final;
     void extensionsInitialized() final;
-    ShutdownFlag aboutToShutdown() final;
 
     class QmlJSEditorPluginPrivate *d = nullptr;
 };