RFR: 8263852: Unused method SoftRefPolicy::use_should_clear_all_soft_refs

[Albert Mingkun Yang]

Trivial change of removing an unused method.

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Commit messages:
 - soft

Changes: https://git.openjdk.java.net/jdk/pull/3085/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3085&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263852
  Stats: 9 lines in 2 files changed: 0 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3085.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3085/head:pull/3085

PR: https://git.openjdk.java.net/jdk/pull/3085