This removes a dead method, makes constant fields constant, and avoids a
confusing macro (which just prevented me from finding the call to
{set_imported_mutable_globals}).

R=manoskouk@chromium.org

Bug: v8:12425
Change-Id: I76de744c273ed9e2e429647a2d26dc163e1f4525
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3406758Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78733}