Remove outdated additional resource link (#31005)

[MayankT10]

Because

The Additional resource link for Rails 7 importmap is outdated/broken and no longer useful.

This PR

• Removes the broken "Additional reading on Rails 7 importmap" link
• Keeps the remaining valid resources intact

Issue

Closes #31005

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

[ManonLef]

@MayankT10 thank you!