RFR: 8364166: Parallel: Remove the use of soft_ref_policy in Full GC

[Albert Mingkun Yang]

Simple removing redundant soft-ref-clearing checking in full-gc, because Parallel already reacts to this particular gc-cause, `_wb_full_gc`.

Test: tier1-3

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Commit messages:
 - pgc-soft-ref

Changes: https://git.openjdk.org/jdk/pull/26496/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26496&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364166
  Stats: 9 lines in 1 file changed: 0 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26496.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26496/head:pull/26496

PR: https://git.openjdk.org/jdk/pull/26496