RFR: Remove the remaining unclean upstream difference

[Aleksey Shipilev]

After this change, the only shared code differences against upstream are the proper GC interface hooks and ProblemLists.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/shenandoah/pull/272/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=272&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/272.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/272/head:pull/272

PR: https://git.openjdk.org/shenandoah/pull/272