tweak iterator_has_known_distance_v
tweak iterator_has_known_distance_v

Summary: No need for separate helper alias `iterator_category_of_t_`.

Reviewed By: Gownta

Differential Revision: D36907778

fbshipit-source-id: c30ddf1f996fde71841af20fc500bf5806236d9f

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly