RFR: Remove the remaining unclean upstream difference

[Kelvin Nilsen]

On Wed, 3 May 2023 10:59:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

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

Marked as reviewed by kdnilsen (Committer).

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

PR Review: https://git.openjdk.org/shenandoah/pull/272#pullrequestreview-1410889254