Remove one of the SparsePRT entry tables

Currently SparsePRT contains two entry tables called _cur and _next.

This has originally been used(?) to allow safe adding of remembered set entries during iteration.

That feature has never been used (at least >= JDK7), so what was left was some complicated dance in various phases to always iterate over the right table, and make sure to release resources correctly.

Remove that feature as it only complicates the code and keeps on memory longer than needed.