RFR: 8315039: Parallel: Remove unimplemented PSYoungGen::oop_iterate

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/15431/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315039
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15431/head:pull/15431

PR: https://git.openjdk.org/jdk/pull/15431