Just use the default operators instead of reimplementing them for
{Steal} and {Split}.

Drive-by: Remove unactionable TODO.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I7556cbf7264cf271b2e8966a5e96ca8e41eb3e73
Reviewed-on: https://chromium-review.googlesource.com/789862
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49640}