Stack limits were additionally maintained in pseudo-smi roots.
"Pseudo", because we stored the raw limit pointers there, just making
sure their values looked like smis by masking the least significant
bits.

This mechanism is no longer needed now that we can access the stack
limit external references as efficiently as the smi roots.

Bug: v8:9534
Change-Id: I37e78851c97aebc1012ea0e95075e56366a40a73
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1745339
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63183}