It's unused and hinders separating read-only roots into a shared table.

Bug: v8:8562
Change-Id: I4a2889d9be5df877b2f9d11ce5146a091d864cd3
Reviewed-on: https://chromium-review.googlesource.com/c/1466963Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#59539}