Remove unused MessagePublisherPrivate forward declaration (backport #570)

[mergify]

Summary

Removed an unused forward declaration.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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This is an automatic backport of pull request #570 done by Mergify.

[mergify]

Cherry-pick of 75b3f72 has failed:

On branch mergify/bp/ign-transport11/pr-570
Your branch is up to date with 'origin/ign-transport11'.

You are currently cherry-picking commit 75b3f72.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   include/gz/transport/Publisher.hh

no changes added to commit (use "git add" and/or "git commit -a")

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