RFR: 8204097: Simplify OopStorage::AllocateList block entry access

[Kim Barrett]

Please review this simplification of OopStorage::AllocateList,
removing the no longer used support for blocks being in multiple lists
simultaneously. There is now only one list of blocks, the
_allocate_list.

CR:
https://bugs.openjdk.java.net/browse/JDK-8204097

Webrev:
http://cr.openjdk.java.net/~kbarrett/8204097/open.00/

Testing:
Mach5 tier{1,2,3}