RFR: 8331924: Parallel: Remove unused MutableSpace::mangle_unused_area_complete

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/19137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331924
  Stats: 10 lines in 4 files changed: 0 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19137/head:pull/19137

PR: https://git.openjdk.org/jdk/pull/19137