This improves the performance of initializing instances with
private methods by 5-6x and makes it on par with initializing
instances with only public methods.

Drive-by: remove the obsolete AddPrivateField runtime function
along with the AddPrivateBrand function.

Bug: v8:10793, v8:12355
Change-Id: Ic0896adb2102db7790f955fecbdd22f61ab2e7ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3256999
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77664}