RFR: 8361426: (ref) Remove jdk.internal.ref.Cleaner

[Viktor Klang]

On Sat, 5 Jul 2025 00:05:39 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change that removes the class jdk.internal.ref.Cleaner.
> It is no longer used after JDK-8344332.
> 
> Testing: mach5 tier1-3

Thanks Kim!

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Marked as reviewed by vklang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26141#pullrequestreview-2997977942