Cleanup - Use RemoveAllChildren() over DisposeAllChildren() (#39848)

* Content - change the (should-be-obsolete) DisposeAllChildren into the more robust RemoveAllChildren.

* Remove duplicate calls.

---------

Co-authored-by: ElectroJr <leonsfriedrich@gmail.com>

Content.Client/Mapping/MappingState.cs

@@ -861,7 +861,7 @@ public sealed class MappingState : GameplayStateBase
         }
         else
         {
-            button.ChildrenPrototypes.DisposeAllChildren();
+            button.ChildrenPrototypes.RemoveAllChildren();
             button.CollapseButton.Label.Text = "▶";
         }
     }