RFR: 8367476: Shenandoah: Remove use of CollectedHeap::_soft_ref_policy [v2]

[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 usage of `CollectedHeap::_soft_ref_policy`, so it can be removed.
> 
> Passes tier(1,2,3)_gc_shenandoah.

Francesco Andreuzzi has updated the pull request incrementally with two additional commits since the last revision:

 - eagerly initialize to always-clear
 - move to gccause

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27239/files
  - new: https://git.openjdk.org/jdk/pull/27239/files/01e41d7b..a55b8bd2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27239&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27239&range=00-01

  Stats: 30 lines in 8 files changed: 11 ins; 12 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