remove "low memory" heuristic from heap code

The original goal was to minimize memory usage by garbage collecting
more frequently and more comprehensively as we got closer to the heap
limit.  In practice, though, this just slowed the VM to a crawl as
memory pressure increased.  If an app really wants to use a lot of
memory, the VM shouldn't penalize it aside from throwing an
OutOfMemoryError if it exceeds the limit.