RFR: 8370326: Parallel: Remove unused ParCompactionManager::push

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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Commit messages:
 - pgc-trivial-unused

Changes: https://git.openjdk.org/jdk/pull/27914/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370326
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27914.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27914/head:pull/27914

PR: https://git.openjdk.org/jdk/pull/27914