Also remove ImportNow() and helper runtime functions and Context methods
that are now unused.

Bug: v8:7624
Change-Id: I109b112d2147240e72eb0ed6112a267057de59cb
Reviewed-on: https://chromium-review.googlesource.com/c/1385224
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58415}