There's now only a single caller to InternalPromiseThen left,
which is the Promise.prototype.then implementation, so there's
no need to have a separate helper function.

Bug: v8:7253
Change-Id: I0e1ea674c942f735dd069137182232f34d16a729
Reviewed-on: https://chromium-review.googlesource.com/897762Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51031}