docs: remove outdated URL shortener guidance

[chenlichao]

Description

Updates CONTRIBUTING.md to remove the exception that allowed Google URL Shortener for long tutorial URLs.

Motivation and Context

The contribution guide already requires tutorial links to point straight to the tutorial and not use URL shorteners. The following sentence made an exception for Google URL Shortener, but Google's official developer blog says goo.gl links are being discontinued and will no longer be available. Removing the exception keeps the guideline consistent and avoids encouraging contributors to submit shortened links.

Reference: https://developers.googleblog.com/en/google-url-shortener-links-will-no-longer-be-available/

How Has This Been Tested?

• Ran git diff --check
• Verified the change is a one-line documentation-only update

Types of changes

• Content Update (change which fixes an issue or updates an already existing submission)
• New Article (change which adds functionality)
• Documentation change

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have made checks to ensure URLs and other resources are valid