RFR: 8328350: G1: Remove DO_DISCOVERED_AND_DISCOVERY

[Guoxiong Li]

On Mon, 18 Mar 2024 11:45:22 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> 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

Looks good. The copyrights of these four files need to be adjusted.

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Marked as reviewed by gli (Committer).

PR Review: https://git.openjdk.org/jdk/pull/18346#pullrequestreview-1946277985