RFR: 8327477: Parallel: Remove _data_location and _highest_ref in ParallelCompactData [v2]

[Albert Mingkun Yang]

> Trivial removing dead code.

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - review
 - trivial

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/18139/files
  - new: https://git.openjdk.org/jdk/pull/18139/files/e7b5fab1..b48d61b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18139&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18139&range=00-01

  Stats: 2113 lines in 37 files changed: 798 ins; 1246 del; 69 mod
  Patch: https://git.openjdk.org/jdk/pull/18139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18139/head:pull/18139

PR: https://git.openjdk.org/jdk/pull/18139