RFR: 8328350: G1: Remove DO_DISCOVERED_AND_DISCOVERY

Mon Mar 18 11:50:50 UTC 2024

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

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Commit messages:
 - g1-remove-DO_DISCOVERED_AND_DISCOVERY

Changes: https://git.openjdk.org/jdk/pull/18346/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328350
  Stats: 22 lines in 4 files changed: 0 ins; 22 del; 0 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