RFR: 8297333: Parallel: Remove unused methods in PCIterateMarkAndPushClosure

[Albert Mingkun Yang]

Trivial change of removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11261/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11261&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297333
  Stats: 17 lines in 2 files changed: 0 ins; 17 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11261.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11261/head:pull/11261

PR: https://git.openjdk.org/jdk/pull/11261