fixtures: remove an incorrect comment

The `request` thing is really accurate. The comment in the docstring is
more accurate, so let's remove this redundant comment.

Author: bluetech

src/_pytest/fixtures.py

@@ -345,11 +345,6 @@ def prune_dependency_tree(self) -> None:
         working_set = set(self.initialnames)
         while working_set:
             argname = working_set.pop()
-            # Argname may be something not included in the original names_closure,
-            # in which case we ignore it. This currently happens with pseudo
-            # FixtureDefs which wrap 'get_direct_param_fixture_func(request)'.
-            # So they introduce the new dependency 'request' which might have
-            # been missing in the original tree (closure).
             if argname not in closure and argname in self.names_closure:
                 closure.add(argname)
                 if argname in self.name2fixturedefs: