Nim: Avoid using the global object pool

Use the pattern that's currently being established.

Change-Id: Ib716b35473fe5060b9ba5beea240ed46cfb1c5c7
Reviewed-by: Filippo Cucchetto <filippocucchetto@gmail.com>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/nim/nimplugin.h

@@ -35,9 +35,8 @@ class NimPlugin : public ExtensionSystem::IPlugin
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "Nim.json")
 
 public:
-    NimPlugin();
-
-    ~NimPlugin();
+    NimPlugin() = default;
+    ~NimPlugin() final;
 
     bool initialize(const QStringList &arguments, QString *errorMessage) final;
     void extensionsInitialized() final;