Remove skip link focus fix from Twenty Twenty-One

[sabernhardt]

• Replaces contents of skip-link-focus-fix.js.
• Outputs an HTML comment if anyone adds twenty_twenty_one_skip_link_focus_fix to the wp_print_footer_scripts action (in a plugin or a child theme).

Trac 64590

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[github-actions]

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Props sabernhardt, joedolson, westonruter.

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[github-actions]

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[joedolson]

Closed in r61635