RFR: 8372269: Parallel: Remove unused ParallelScavengeHeap::base

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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Commit messages:
 - pgc-trivial-remove

Changes: https://git.openjdk.org/jdk/pull/28427/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28427&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372269
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28427.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28427/head:pull/28427

PR: https://git.openjdk.org/jdk/pull/28427