RFR: 8300958: Parallel: Remove unused MutableNUMASpace::capacity_in_words

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-remove-dead

Changes: https://git.openjdk.org/jdk/pull/12162/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12162&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300958
  Stats: 20 lines in 3 files changed: 0 ins; 19 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12162.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12162/head:pull/12162

PR: https://git.openjdk.org/jdk/pull/12162