Drop GtkCallback

It is no longer used in any of our public APIs.

docs/reference/gtk/gtk4-sections.txt

@@ -4406,7 +4406,6 @@ gtk_widget_paintable_get_type
 <TITLE>GtkWidget</TITLE>
 GtkWidget
 GtkWidgetClass
-GtkCallback
 GtkRequisition
 GtkAllocation
 gtk_widget_new

gtk/gtkwidget.h

@@ -70,17 +70,6 @@ typedef struct _GtkWidgetClassPrivate  GtkWidgetClassPrivate;
  */
 typedef         GdkRectangle       GtkAllocation;
 
-/**
- * GtkCallback:
- * @widget: the widget to operate on
- * @data: (closure): user-supplied data
- *
- * The type of the callback functions used for e.g. iterating over
- * the children of a container, see gtk_container_foreach().
- */
-typedef void    (*GtkCallback)     (GtkWidget        *widget,
-                                    gpointer          data);
-
 /**
  * GtkTickCallback:
  * @widget: the widget

gtk/gtkwidgetprivate.h

@@ -306,6 +306,9 @@ void              gtk_widget_adjust_baseline_request       (GtkWidget *widget,
                                                             int       *minimum_baseline,
                                                             int       *natural_baseline);
 
+typedef void    (*GtkCallback)     (GtkWidget        *widget,
+                                    gpointer          data);
+
 void              gtk_widget_forall                        (GtkWidget            *widget,
                                                             GtkCallback           callback,
                                                             gpointer              user_data);