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/emacskeys/emacskeysplugin.cpp

@@ -135,11 +135,6 @@ void EmacsKeysPlugin::extensionsInitialized()
 {
 }
 
-ExtensionSystem::IPlugin::ShutdownFlag EmacsKeysPlugin::aboutToShutdown()
-{
-    return SynchronousShutdown;
-}
-
 void EmacsKeysPlugin::editorAboutToClose(IEditor *editor)
 {
     auto w = qobject_cast<QPlainTextEdit*>(editor->widget());

src/plugins/emacskeys/emacskeysplugin.h

@@ -57,7 +57,6 @@ public:
 
     void initialize() override;
     void extensionsInitialized() override;
-    ShutdownFlag aboutToShutdown() override;
 
 private:
     void editorAboutToClose(Core::IEditor *editor);