RFR: 8283791: Parallel: Remove unnecessary condition in PSKeepAliveClosure

[Albert Mingkun Yang]

Simple change of removing an unnecessary if-check.

Test: tier1-3, hotspot_gc

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Commit messages:
 - pgc-weak-keep-alive

Changes: https://git.openjdk.java.net/jdk/pull/7997/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7997&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283791
  Stats: 9 lines in 1 file changed: 2 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7997.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7997/head:pull/7997

PR: https://git.openjdk.java.net/jdk/pull/7997