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/UserInterface/Controls/SplitBar.xaml.cs

@@ -1,4 +1,4 @@
-using System.Numerics;
+using System.Numerics;
 using Robust.Client.AutoGenerated;
 using Robust.Client.Graphics;
 using Robust.Client.UserInterface.Controls;
@@ -19,7 +19,7 @@ namespace Content.Client.UserInterface.Controls
 
         public void Clear()
         {
-            DisposeAllChildren();
+            RemoveAllChildren();
         }
 
         public void AddEntry(float amount, Color color, string? tooltip = null)