RFR: 8302661: Parallel: Remove PSVirtualSpace::is_aligned

[Albert Mingkun Yang]

Simple removing redundant code.

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Commit messages:
 - pgc-is-aligned

Changes: https://git.openjdk.org/jdk/pull/12592/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12592&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302661
  Stats: 14 lines in 2 files changed: 1 ins; 7 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12592.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12592/head:pull/12592

PR: https://git.openjdk.org/jdk/pull/12592