RFR: 8307100: Remove ReferentBasedDiscovery reference discovery policy [v2]

[Albert Mingkun Yang]

> Mostly consisting of mechanic refactoring after replacing `RefDiscoveryPolicy == ...` with `true` or `false`.
> 
> Test: tier1-6

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - merge
 - remove-referent-policy

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Changes: https://git.openjdk.org/jdk/pull/13715/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13715&range=01
  Stats: 87 lines in 6 files changed: 2 ins; 68 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/13715.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13715/head:pull/13715

PR: https://git.openjdk.org/jdk/pull/13715