Administration: Combine the styles for `<p>` and `.wp-die-message`, instead of duplicating.

This brings some consistency with `_default_wp_die_handler()`, where they are already combined.

Follow-up to [45909].

See #47580.
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git-svn-id: https://core.svn.wordpress.org/trunk@47849 1a063a9b-81f0-0310-95a4-ce76da25c4cd