RFR: Remove legacy section on mercurial [v2]

[Iris Clark]

On Fri, 14 Jun 2024 00:17:51 GMT, Jesper Wilhelmsson <jwilhelm at openjdk.org> wrote:

>> Mercurial is no longer used in the OpenJDK project.
>
> Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   One more place

Happy to see the Hg content removed!

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Marked as reviewed by iris (Reviewer).

PR Review: https://git.openjdk.org/guide/pull/127#pullrequestreview-2117295723