Also remove the unused AllocateTwoByteInternalizedString method.

Change-Id: I28e2c39a0196c48e56942efc263009aa3676cdec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3714988Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81310}