[gtk/mcatanzaro/unparent] Remove incorrect unparenting guidance from migration guide

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 750b718bf0b7991764c2f2fcdc5b8a0dfffd7495
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Thu Jan 7 09:42:08 2021 -0600

    Remove incorrect unparenting guidance from migration guide
    
    This rule is almost always wrong, and we're having some trouble agreeing
    on replacement text. Let's remove the whole paragraph for now, to avoid
    confusion.

 docs/reference/gtk/migrating-3to4.md | 8 --------
 1 file changed, 8 deletions(-)
---
diff --git a/docs/reference/gtk/migrating-3to4.md b/docs/reference/gtk/migrating-3to4.md
index 9112ec2132..aa93f4ac1d 100644
--- a/docs/reference/gtk/migrating-3to4.md
+++ b/docs/reference/gtk/migrating-3to4.md
@@ -264,14 +264,6 @@ therefore can no longer be used to break reference cycles. A typical sign
 of a reference cycle involving a toplevel window is when closing the window
 does not make the application quit.
 
-A good rule to follow is: If you set a widget pointer with
-gtk_widget_class_bind_template_child() in class_init(), you need to
-unparent it in dispose(). The slight complication here is that you need
-to respect the widget hierarchy while doing so. Ie if you set both `field1`
-and `field2`, but `field1` is an ancestor of `field2`, then you only need
-to unparent `field1` — doing so will remove the the entire subtree below
-`field1`, including `field2`.
-
 ### Stop using GdkScreen
 
 The GdkScreen object has been removed in GTK 4. Most of its APIs already