RFR: 8331284: Inline methods in softRefPolicy.cpp

[Albert Mingkun Yang]

Trivial moving methods from cpp to hpp. `virtual` is removed because this class has no subclasses.

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Commit messages:
 - remove-soft-ref-policy

Changes: https://git.openjdk.org/jdk/pull/18993/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18993&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331284
  Stats: 41 lines in 2 files changed: 4 ins; 35 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18993.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18993/head:pull/18993

PR: https://git.openjdk.org/jdk/pull/18993