RFR: 8369277: Remove unused CodeCacheUnloadingTask::_num_workers

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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Commit messages:
 - remove-field-trivial

Changes: https://git.openjdk.org/jdk/pull/27671/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27671&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369277
  Stats: 6 lines in 4 files changed: 0 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27671.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27671/head:pull/27671

PR: https://git.openjdk.org/jdk/pull/27671