With the introduction of the StringForwardingTable, there are no
concurrent transitions of strings happening anymore.

- Remove String migration sentinel maps + helper methods
- Remove CanMigrateInParallel()
- Remove MigrateStringMapUnderLockIfNeeded() and simplify MakeThin()

There is still unused code I didn't remove in this CL, as we might need
it later for shared struct features: YIELD_PROCESSOR for spinlocks and
Relaxed_Memcmp().

Bug: v8:12007
Change-Id: Iaa09ef93d2ee612e42cd73395a06eada22fe7dae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3629545
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80414}