RFR: 8302864: Parallel: Remove PSVirtualSpace::pointer_delta

[Albert Mingkun Yang]

Simple removing redundant code.

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Commit messages:
 - pgc-pointer-delta

Changes: https://git.openjdk.org/jdk/pull/12657/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12657&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302864
  Stats: 9 lines in 1 file changed: 0 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12657.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12657/head:pull/12657

PR: https://git.openjdk.org/jdk/pull/12657