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

[Kim Barrett]

Please review this change that removes the class jdk.internal.ref.Cleaner.
It is no longer used after JDK-8344332.

Testing: mach5 tier1-3

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Commit messages:
 - remove jdk.internal.ref.Cleaner

Changes: https://git.openjdk.org/jdk/pull/26141/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361426
  Stats: 233 lines in 3 files changed: 0 ins; 232 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26141.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26141/head:pull/26141

PR: https://git.openjdk.org/jdk/pull/26141