OopStorage::AllocateList supports having blocks in multiple lists. This is a left-over from the original implementation, when blocks were in both the _allocate_list and the _active_list. However,
JDK-8200557 eliminated the _active_list, so blocks are now only ever in (at most) one list, the _allocate_list. This allows the implementation to be simplified, eliminating the multi-list support.