After f1a55180, we no longer have to
deal with a case where we rewrite destructuring assignments belonging
to some inner function. Remove a bit of code and add DCHECKs to reflect
that fact.

Change-Id: I7553e86d4ae2f6290853eee38f85d2f243b778a9
Reviewed-on: https://chromium-review.googlesource.com/922893Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51404}