RFR: 8332678: Serial: Remove use of should_clear_all_soft_refs in serial folder

[Albert Mingkun Yang]

Simple removing all accesses to `soft_ref_policy` in Serial code, because the decision to clear-soft-refs is made inside Serial and captured in local variables.

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Commit messages:
 - s1-clear-soft-ref

Changes: https://git.openjdk.org/jdk/pull/19346/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332678
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19346.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19346/head:pull/19346

PR: https://git.openjdk.org/jdk/pull/19346