Remove obsolete doc (#86415)

packages/flutter/lib/src/material/dialog.dart

@@ -958,8 +958,6 @@ Widget _buildMaterialDialogTransitions(BuildContext context, Animation<double> a
 /// `showDialog` is originally called from. Use a [StatefulBuilder] or a
 /// custom [StatefulWidget] if the dialog needs to update dynamically.
 ///
-/// The `child` argument is deprecated, and should be replaced with `builder`.
-///
 /// The `context` argument is used to look up the [Navigator] and [Theme] for
 /// the dialog. It is only used when the method is called. Its corresponding
 /// widget can be safely removed from the tree before the dialog is closed.