[Fonts API] Removes adding missing fonts into theme.json

[hellofromtonya]

Fixes #51716

What?

Removes adding the missing fonts into the theme.json by removing WP_Fonts_Resolver:: add_missing_fonts_to_theme_json() and its associated code.

Why?

Previously the Fonts API added the fonts from plugins into theme.json. With the new Font Library and font management, plugins will no longer register with the Fonts API, but instead will interact with the Font Library (when that capability is available).

How?

• Remove invoking it from within WP_Theme_JSON_Gutenberg::get_theme_data().
• Remove WP_Fonts_Resolver:: add_missing_fonts_to_theme_json() and any of its associated private methods.
• Remove its tests.

Testing Instructions

Screenshots or screencast

[github-actions]

Flaky tests detected in d1e5584.
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/5327457174
📝 Reported issues:

• [Flaky Test] Pick the additional variation in the inserted Columns block #47057 in specs/editor/plugins/block-variations.test.js
• [Flaky Test] should convert to a preformatted block #51607 in /test/e2e/specs/editor/various/convert-block-type.spec.js

[github-actions]

Warning: Type of PR label error

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• Type-related labels to choose from: [Type] Accessibility (a11y), [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
• Labels found: [Feature] Typography.

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[hellofromtonya]

The Fonts API is deprecated, replaced by Font Face to work with the Font Library. Thus this PR can be closed without merging.