RFR: 8261799: Remove unnecessary cast in psParallelCompact.hpp

[Leo Korinth]

Unnecessary casts confuses me.

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Commit messages:
 - 8261799: Remove unnessesary cast in psParallelCompact.hpp

Changes: https://git.openjdk.java.net/jdk/pull/2628/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2628&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261799
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2628.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2628/head:pull/2628

PR: https://git.openjdk.java.net/jdk/pull/2628