Provide an empty default IPlugin::extensionsInitialized()

And remove all empy re-implementations.

Change-Id: I19f0b4e55c042c96693ecb89766f97f0a97b54ae
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/android/androidplugin.h

@@ -38,7 +38,6 @@ class AndroidPlugin final : public ExtensionSystem::IPlugin
     ~AndroidPlugin() final;
 
     bool initialize(const QStringList &arguments, QString *errorMessage) final;
-    void extensionsInitialized() final { }
 
     void kitsRestored();