This reverts commit 3d1d8eae.

Reason for revert: Speculative revert; seems to break all nosnap bots, e.g. https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20nosnap/25240

Original change's description:
> [heap] Skip ro-space from heap iterators, add CombinedHeapIterator.
> 
> Read-only space sharing requires an iterator independent of heap. This
> also enables future removal of read-only space from heap.
> 
> Bug: v8:7464
> Change-Id: Ia07a9369494ea2c547d12c01ffa1d7b8b6bbeabc
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1552795
> Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Dan Elphick <delphick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#60819}

TBR=ulan@chromium.org,hpayer@chromium.org,delphick@chromium.org,goszczycki@google.com

Change-Id: I64b58b1b0c5eb073a6d2cfae81bb4de65f0511bf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7464
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1565895Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60825}