inspector: Drop unused functions

The entire UI is constructed with templates, so the wrapper
constructors are never called, except for gtk_inspector_window_new,
which gets called from the GTK+ code.

gtk/inspector/widget-tree.h

@@ -55,7 +55,6 @@ G_BEGIN_DECLS
 
 
 GType      gtk_inspector_widget_tree_get_type            (void);
-GtkWidget *gtk_inspector_widget_tree_new                 (void);
 
 GObject   *gtk_inspector_widget_tree_get_selected_object (GtkInspectorWidgetTree *wt);