STYLE: use direct iteration for HashSet

- The iterator for a HashSet dereferences directly to its key.

- Eg,

      for (const label patchi : patchSet)
      {
          ...
      }
  vs.
      forAllConstIter(labelHashSet, patchSet, iter)
      {
          const label patchi = iter.key();
          ...
      }

src/functionObjects/utilities/ensightWrite/ensightWrite.C

@@ -244,9 +244,9 @@ bool Foam::functionObjects::ensightWrite::write()
         }
     }
 
-    forAllConstIter(wordHashSet, candidates, iter)
+    for (const word& cand : candidates)
     {
-        process(iter.key());
+        process(cand);
     }
 
     Log << " )" << endl;