Also delete undefined ContextRef methods and make
Context::set_previous private (it is only used when
creating a new context).

Bug: v8:7790
Change-Id: I25a701f317f0f4e82432f7537eec1d63c5ef63f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2886860
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74521}