Remove unused group links index route

What does this MR do?

Projects::GroupLinks#index(project_group_links_path) redirects to namespace_project_project_members_path.

This route would never work since it needs the :namespace_id and :project_id to be valid and those are not passed. build_canonical_path will handle the addition of :namespace_id, but would at least need the project passed.

This seems to be only referenced in a test, so we'll modify that test for another path and remove this route.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #218700