Auto merge of #18107 - mrobinson:remove-pseudo-scrolling-area, r=emilio

Remove unused StackingContextType

This hasn't been in use for quite some time.

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components/gfx/display_list/mod.rs

@@ -399,7 +399,6 @@ pub enum StackingContextType {
     Real,
     PseudoPositioned,
     PseudoFloat,
-    PseudoScrollingArea,
 }
 
 #[derive(Clone, HeapSizeOf, Deserialize, Serialize)]