Font Library: Remove slug from collection schema to accommodate the changes on the wp_register_font_collection function signature

[matiasbenedetto]

What?

Font Library: Remove slug from collection schema to accommodate the changes on the wp_register_font_collection function signature done in #58530

Why?

The slug is no longer part of the required collection JSON data. The slug is added in PHP when a font collection is registered. Example

wp_register_font_collection( 'my-collection-slug', 'https://example.com/fonts/my-collection.json' );

How?

Removing the slug from the schema

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Props: mmaattiiaass, pbking.

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Co-authored-by: matiasbenedetto <mmaattiiaass@git.wordpress.org>
Co-authored-by: pbking <pbking@git.wordpress.org>

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