RFR: 8364254: Serial: Remove soft ref policy update in WhiteBox FullGC

[Albert Mingkun Yang]

Simple removing Serial specific logic in `WB_FullGC`, because Serial full-gc handles white-box full-gc already.

Test: tier1-3

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Commit messages:
 - sgc-whitebox

Changes: https://git.openjdk.org/jdk/pull/26528/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26528&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364254
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26528.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26528/head:pull/26528

PR: https://git.openjdk.org/jdk/pull/26528