RFR: 8290357: Drop HeapRegion::marked_bytes()

[Thomas Schatzl]

Hi all,

  please review this removal of the `HeapRegion::_marked_bytes` member that records the bytes marked below tams in the recent mark as it is not really interesting in the removed places. I added comments to give reasons for this removal in the particular places for your review.

Testing: jtreg gc/g1, gha

Thanks,
  Thomas

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Commit messages:
 - initial version, use live_bytes instead of marked_bytes

Changes: https://git.openjdk.org/jdk/pull/9511/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9511&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290357
  Stats: 47 lines in 8 files changed: 0 ins; 36 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/9511.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9511/head:pull/9511

PR: https://git.openjdk.org/jdk/pull/9511