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/clangcodemodel/clangcodemodelplugin.h

@@ -45,7 +45,6 @@ class ClangCodeModelPlugin final: public ExtensionSystem::IPlugin
 public:
     ~ClangCodeModelPlugin() override;
     bool initialize(const QStringList &arguments, QString *errorMessage) override;
-    void extensionsInitialized() override;
 
 private:
     void maybeHandleBatchFileAndExit() const;