migrating-3to4: Don't mention removed Box method &

donʼt mention its renamed successor either, as that has its own section
later. We could have another sentence paragraph like ‘In the case of
GtkBox, the pack methods have been renamed to X and lost the trailing
arguments Y’, but that wonʼt help people prepare still on GTK3, which is
the point in the affected section… so just remove the misleading relic.

docs/reference/gtk/migrating-3to4.md

@@ -258,7 +258,7 @@ Instead of implementing GtkWidget.destroy, you can implement GObject.dispose.
 GTK 4 removes `gtk_container_add()` and `gtk_container_remove()`. While there
 is not always a replacement for `gtk_container_remove()` in GTK 3, you can
 replace many uses of `gtk_container_add()` with equivalent container-specific
-APIs such as `gtk_box_pack_start()` or `gtk_grid_attach()`, and thereby reduce
+APIs such as `gtk_grid_attach()`, and thereby reduce
 the amount of work you have to do at the time of the switch.
 
 ### Review your use of icon resources