RFR: 8309403: Serial: Remove the useless adaptive size policy in GenCollectedHeap [v2]

[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

Guoxiong Li has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Restore the _should_clear_all_soft_refs in WB_FullGC.
 - Merge branch 'master' into JDK-8309403
 - JDK-8309403

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14296/files
  - new: https://git.openjdk.org/jdk/pull/14296/files/ba3c1066..1922cdfe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14296&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14296&range=00-01

  Stats: 8 lines in 2 files changed: 0 ins; 2 del; 6 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