RFR: 8302886: Parallel: Remove unimplemented methods in ParCompactionManager

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-trivial

Changes: https://git.openjdk.org/jdk/pull/12672/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12672&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302886
  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12672.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12672/head:pull/12672

PR: https://git.openjdk.org/jdk/pull/12672