fix: Remove additional resources section from React Testing and Ecosystem lessons

[bittoby]

Because

The additional resources sections in several React Testing and Ecosystem lessons contain supplemental links that are no longer needed.

This PR

• Remove the "Additional resources" section from the Introduction to React Testing lesson
• Remove the "Additional resources" section from the React Router lesson
• Remove the "Additional resources" section from the Fetching Data in React lesson
• Remove the "Additional resources" section from the Styling React Applications lesson

Issue

Closes #30918

Additional Information

No additional information.

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide