Some helpers in array.js are now dead code after the recent changes
to array builtins (splice/unshift).

R=jgruber@chromium.org

Bug: v8:8015
Change-Id: I3e0c0280516b539533bcc3b21d84e448112e4ec7
Reviewed-on: https://chromium-review.googlesource.com/1208632Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#55678}