Python: Write DataFlow::update more succinctly

This has no impact on performance, but it cleans up the code a bit,
and (hopefully) makes it more readable.

python/ql/src/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -215,10 +215,7 @@ private predicate localEssaStep(EssaNode nodeFrom, EssaNode nodeTo) {
  * Holds if `result` is either `node`, or the post-update node for `node`.
  */
 private Node update(Node node) {
-  exists(PostUpdateNode pun |
-    node = pun.getPreUpdateNode() and
-    result = pun
-  )
+  node = result.(PostUpdateNode).getPreUpdateNode()
   or
   result = node
 }