OnStackReplacement itself was dead code.

Bug: v8:6666
Change-Id: I72df335f23fb749e652899a170bb3dc800992ba7
Reviewed-on: https://chromium-review.googlesource.com/1232635
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56039}