RFR: 8309403: Serial: Remove the useless adaptive size policy in GenCollectedHeap

[Guoxiong Li]

Hi all,

This patch removes the useless code related to the `AdaptiveSizePolicy` in `GenCollectedHeap`.
As a result, the `SoftRefGenPolicy` is unnecessary too, use `SoftRefPolicy` instead.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8309403

Changes: https://git.openjdk.org/jdk/pull/14296/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14296&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309403
  Stats: 114 lines in 5 files changed: 0 ins; 110 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14296.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14296/head:pull/14296

PR: https://git.openjdk.org/jdk/pull/14296