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

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

* Remove duplicate calls.

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

Content.Client/Lobby/UI/CharacterSetupGui.xaml.cs

@@ -72,7 +72,7 @@ namespace Content.Client.Lobby.UI
         public void ReloadCharacterPickers()
         {
             _createNewCharacterButton.Orphan();
-            Characters.DisposeAllChildren();
+            Characters.RemoveAllChildren();
 
             var numberOfFullSlots = 0;
             var characterButtonsGroup = new ButtonGroup();