RFR: 8332871: Parallel: Remove public bits APIs in ParMarkBitMap

[Albert Mingkun Yang]

Trivial unifying APIs to operate only on `HeapWord*`.

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Commit messages:
 - pgc-bit-api

Changes: https://git.openjdk.org/jdk/pull/19388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332871
  Stats: 30 lines in 3 files changed: 6 ins; 16 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/19388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19388/head:pull/19388

PR: https://git.openjdk.org/jdk/pull/19388