Docs: Remove PHPDoc for non-existing parameter

[Soean]

What?

The function gutenberg_override_global_styles_endpoint only has a $args parameter, so we should remove the PHPDoc for the non-existing $post_type parameter.

[github-actions]

Flaky tests detected in a95b778.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/11030003963
📝 Reported issues:

• [Flaky Test] Page title is updated 2 #65520 in /test/e2e/specs/interactivity/router-regions.spec.ts

[Mamaduka]

Thank you, @Soean!

[github-actions]

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Co-authored-by: Soean <soean@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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