RFR: 8340573: Remove unused G1ParScanThreadState::_partial_objarray_chunk_size

[Kim Barrett]

Please review this trivial change to remove unused G1ParScanThreadState::_partial_objarray_chunk_size.

Testing: local (linux-x64) clean build

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Commit messages:
 - remove unused _partial_objarray_chunk_size

Changes: https://git.openjdk.org/jdk/pull/21117/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21117&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340573
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21117.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21117/head:pull/21117

PR: https://git.openjdk.org/jdk/pull/21117