RFR: 8367476: Shenandoah: Remove use of CollectedHeap::_soft_ref_policy

[Francesco Andreuzzi]

Similarly to what was done in #26496 and #26648, the usages of `CollectedHeap::_soft_ref_policy` in Shenandoah can be replaced with an earlier call to ShenandoahReferenceProcessor::set_soft_reference_policy.

This is the last usages of `CollectedHeap::_soft_ref_policy`, so it can be removed.

Passes tier(1,2,3)_gc_shenandoah.

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Commit messages:
 - can be const
 - stuff
 - stuff

Changes: https://git.openjdk.org/jdk/pull/27239/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367476
  Stats: 97 lines in 17 files changed: 16 ins; 74 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/27239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27239/head:pull/27239

PR: https://git.openjdk.org/jdk/pull/27239