This mutex wasn't really used anymore. This should also speed up
code object allocation a bit.

Bug: v8:11888
Change-Id: I8ddc2ecc1aec74e8eb3e2d4b96354c50f3bff350
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2966382Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75185}