GH-2859 remove twitch page and modpack import from URL

The functionality was broken, beyond repair and an ongoing maintenance
nightmare.

application/dialogs/NewInstanceDialog.h

@@ -29,7 +29,6 @@ class NewInstanceDialog;
 class PageContainer;
 class QDialogButtonBox;
 class ImportPage;
-class TwitchPage;
 
 class NewInstanceDialog : public QDialog, public BasePageProvider
 {
@@ -68,7 +67,6 @@ private:
 
     QString InstIconKey;
     ImportPage *importPage = nullptr;
-    TwitchPage *twitchPage = nullptr;
     std::unique_ptr<InstanceTask> creationTask;
 
     bool importIcon = false;