RFR: 8365237: Remove unused SoftRefPolicy::_all_soft_refs_clear

[Albert Mingkun Yang]

Trivial removing effectively dead code.

Test: tier1

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Commit messages:
 - soft-ref-cleanup

Changes: https://git.openjdk.org/jdk/pull/26722/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26722&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365237
  Stats: 34 lines in 3 files changed: 0 ins; 32 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26722.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26722/head:pull/26722

PR: https://git.openjdk.org/jdk/pull/26722