Since the mutability of HeapNumbers is determined by their owning
object's descriptor array, we can remove the MutableHeapNumber type
entirely, at the cost of a few fewer DCHECKs and a couple of TODOs
to use the descriptor array information.

This is a necessary step towards a follow-up which allows in-place
Double -> Tagged transitions

Design doc: https://docs.google.com/document/d/1VeKIskAakxQFnUBNkhBmVswgR7Vk6T1kAyKRLhqerb4/

Bug: v8:9606
Change-Id: I13209f9c86f1f204088f6fd80089e17d956b4a50
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1743972
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63294}