Since each Array literal containing a spread is individually queued for
rewriting, there's no need for an AstVisitor here: a simple linear
pass through the queue is sufficient.

This patch deletes AstExpressionRewriter and all the machinery supporting
it in the AST. This code was built with the idea of using it as
a general expression rewriting mechanism in the parser, but those use
cases never materialized, and Array spread remains the only thing
that used this code.

Bug: v8:6092
Change-Id: I754c4883099e840881b005f20216f86e57721d5a
Reviewed-on: https://chromium-review.googlesource.com/765051Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49337}