gh-144054: shutdown fix for deferred ref counting (GH-144055)

When shutting down, disable deferred refcounting for all GC objects. It
is important to do this also for untracked objects, which before this
change were getting missed.

Small code cleanup:
We can remove the shutdown case disable_deferred_refcounting() call
inside scan_heap_visitor() if we are careful about it.  The key is
that frame_disable_deferred_refcounting() might fail if the object
is untracked.