Since the interpreter cannot call out to JS any more, there cannot be
more than one activation at a time. Hence remove the concept of
activations.

R=ahaas@chromium.org

Bug: v8:10389
Change-Id: Ifda5624e192464a1aed2943787bc6860d1917719
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2219942Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68118}