RFR: 8347431: Update ObjectMonitor comments

[Coleen Phillimore]

Please review this change that removes a couple of comments that are not true (verified with testing metadata at offset 8).  There are no functional changes.

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Commit messages:
 - 8347431: Update ObjectMonitor comments

Changes: https://git.openjdk.org/jdk/pull/23042/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23042&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347431
  Stats: 12 lines in 1 file changed: 0 ins; 10 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23042.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23042/head:pull/23042

PR: https://git.openjdk.org/jdk/pull/23042