RFR: 8293209: Parallel: Remove unused variables in PSParallelCompact::invoke

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

-------------

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/10118/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10118&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293209
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10118.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10118/head:pull/10118

PR: https://git.openjdk.org/jdk/pull/10118