revset-children: remove usage of `set()`

All smartset classes have fast lookup, so this function will be removed soon.

mercurial/revset.py

@@ -572,7 +572,7 b' def children(repo, subset, x):'
     """``children(set)``
     Child changesets of changesets in set.
     """
-    s = getset(repo, baseset(repo), x).set()
+    s = getset(repo, baseset(repo), x)
     cs = _children(repo, subset, s)
     return subset & cs