RFR: 8343929: Remove PreservedMarksSet::createTask() after JDK-8305895

[Thomas Schatzl]

Hi all,

  please review this trivial cleanup after pushing the Compact Object Header JEP (JDK-8305895).

The method mentioned is unused.

Testing: gha, local compilation

Thanks,
  Thomas

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Commit messages:
 - 8343929

Changes: https://git.openjdk.org/jdk/pull/22006/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22006&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343929
  Stats: 6 lines in 2 files changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22006.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22006/head:pull/22006

PR: https://git.openjdk.org/jdk/pull/22006