This was a pair of a set of maps and their common instance type (if
any), but the instance type field was only used in a printing function.
Removing the whole class in favor of ZoneHandleSet<Map> means we avoid
looking at the heap to determine the common instance type. Eventually
we can use the broker to do this if we need to.

Bug: v8:7790
Change-Id: If0cadf9b17e3b9e77cffc4f0b69e2585aff7c85c
Reviewed-on: https://chromium-review.googlesource.com/c/1481214Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59834}