Simplify the allocation stats accounting

We were doing it in two different ways and asserting that the results
were the same.  In most cases they were, but I found one case where
they weren't: the GC itself allocates some memory for running
finalizers, and this memory was accounted for one way but not the
other.

It was simpler to remove the old way of counting allocation that to
try to fix it up, so I did that.