RFR: 8302674: Parallel: Remove unused methods in MutableNUMASpace

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-trivial

Changes: https://git.openjdk.org/jdk/pull/12598/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302674
  Stats: 79 lines in 2 files changed: 0 ins; 77 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12598.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12598/head:pull/12598

PR: https://git.openjdk.org/jdk/pull/12598