RFR: 8321273: Parallel: Remove unused UpdateOnlyClosure::_space_id

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-remove-space-id

Changes: https://git.openjdk.org/jdk/pull/16950/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16950&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321273
  Stats: 2 lines in 2 files changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16950.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16950/head:pull/16950

PR: https://git.openjdk.org/jdk/pull/16950