RFR: 8328350: G1: Remove DO_DISCOVERED_AND_DISCOVERY [v2]

[Albert Mingkun Yang]

> Remove a special ref-processing iteration mode for G1, because non-null `_discovered` field and requiring ref-discovery should never occur at the same time.
>  
> Test: tier1-6

Albert Mingkun Yang 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 two additional commits since the last revision:

 - year
 - g1-remove-DO_DISCOVERED_AND_DISCOVERY

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18346/files
  - new: https://git.openjdk.org/jdk/pull/18346/files/5dfd1060..9ef446b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18346&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18346&range=00-01

  Stats: 4520 lines in 216 files changed: 1630 ins; 1794 del; 1096 mod
  Patch: https://git.openjdk.org/jdk/pull/18346.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18346/head:pull/18346

PR: https://git.openjdk.org/jdk/pull/18346