(Temporarily) Revert "[dce] clean up `@:used` on types too"

See https://github.com/HaxeFoundation/haxe/issues/7805#issuecomment-464929875

This reverts commit ae0ab793ad40a935e70c66464952210ec4b3e8c1.

src/optimization/dce.ml

@@ -886,8 +886,4 @@ let run com main mode =
 		| x :: l -> x :: remove_meta m l
 	in
 	List.iter (fun cf -> cf.cf_meta <- remove_meta Meta.Used cf.cf_meta) dce.marked_fields;
-	List.iter (fun cf -> cf.cf_meta <- remove_meta Meta.MaybeUsed cf.cf_meta) dce.marked_maybe_fields;
-	List.iter (fun mt ->
-		let mt = t_infos mt in
-		mt.mt_meta <- remove_meta Meta.Used mt.mt_meta
-	) com.types
+	List.iter (fun cf -> cf.cf_meta <- remove_meta Meta.MaybeUsed cf.cf_meta) dce.marked_maybe_fields