Feeds: Remove deprecated wfw namespace.

[xFanly]

Description

Remove the deprecated wfw XML namespace from generated feeds.

The referenced wellformedweb.org domain is no longer maintained.

Trac ticket:
https://core.trac.wordpress.org/ticket/65830

Testing

Verified generated feeds no longer output the deprecated namespace.

[github-actions]

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Props fanly.

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[github-actions]

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[mukeshpanchal27]

Thanks for the PR!

See my comment https://core.trac.wordpress.org/ticket/65830#comment:5