ExtensionSystem: Remove per-plugin object pools

Remove now-unused IPlugin::addAutoReleasedObject and IPlugin::
{add,remove}Object convenience functions that were only forwarding
to the global pool.

Adjust all related tests.

All previous users of these convenience functions are gone, and we do
not want to encourage the use of object pool anymore.

Plugins that wish to share objects to implement weak dependencies
can use the global object pool via  ExtensionSystem::PluginManager::
{add,remove}Object directly.

Change-Id: Ic668ad5504af76963f6d4c69ae160438efc70db5
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/iplugin.h

@@ -64,10 +64,6 @@ public:
 
     PluginSpec *pluginSpec() const;
 
-    void addObject(QObject *obj);
-    void addAutoReleasedObject(QObject *obj);
-    void removeObject(QObject *obj);
-
 signals:
     void asynchronousShutdownFinished();