RFR: 8261356: Clean up enum G1Mark [v2]

[Albert Mingkun Yang]

On Tue, 9 Feb 2021 15:26:19 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> I hoped you would make use of that feature

I didn't know that feature before. Would try using it in future reviews.

The CI was green before the last commit (rename only), so I am integrating it without re-running the CI.

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PR: https://git.openjdk.java.net/jdk/pull/2461