The instance finalizer does nothing useful any more. Freeing the native
allocations is already done in the destructor of the Managed.

R=titzer@chromium.org

Bug: chromium:875647
Change-Id: I154b9da74cf377c2803f66dc959edb4837c6b766
Reviewed-on: https://chromium-review.googlesource.com/1221215Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55849}