RFR: 8321214: Parallel: Remove unused SpaceInfo::_min_dense_prefix

[Zhengyu Gu]

A trivial cleanup that removed unused `_min_dense_prefix` member from `SpaceInfo`.

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Commit messages:
 - 8321214: Parallel: Remove unused SpaceInfo::_min_dense_prefix

Changes: https://git.openjdk.org/jdk/pull/16932/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16932&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321214
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16932.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16932/head:pull/16932

PR: https://git.openjdk.org/jdk/pull/16932