RFR: 8337415: Remove inappropriate Atomic access in FreeListAllocator

[Kim Barrett]

Please review this change to ~FreeListAllocator to replace an inappropriate
use of Atomic::load with a plain access.

Testing: mach5 tier1

-------------

Commit messages:
 - free-list minor cleanups

Changes: https://git.openjdk.org/jdk/pull/20382/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20382&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337415
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20382.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20382/head:pull/20382

PR: https://git.openjdk.org/jdk/pull/20382