Removed long deprecated DNSMultiCast class

[StefanSpieker]

This was removed several years ago, last change was PR#5720 in 2021.

1. According to deprecated-usage-in-plugins, this is not used by any plugin - unfortunately, last build was in September 2024. Would be nice to get this up and running again.
2. According to GitHub search, this is unused.

So I think it is fine to finally delete the class.

Testing done

I just ran a local build and started it up locally.

Proposed changelog entries

• removed almost empty long deprecated DNSMultiCast class

Proposed changelog category

/label removed

Proposed upgrade guidelines

N/A

Submitter checklist

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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[NotMyFault]

According to deprecated-usage-in-plugins, this is not used by any plugin - unfortunately, last build was in September 2024. Would be nice to get this up and running again.

I've triggered a new build.

[A1exKH]

LGTM.

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!