Clean up use of old URL import code

With the rollout of import_by_url_new_page we should no longer be supporting the old url import path.

Ahead of deleting the feature flag, we need to update the code and documentation to point exclusively to the new url import form. This is an intermediary step before deleting the FF.

This could require:

• backend changes in url_controller.rb, _import_project_pane.html.haml
• updating the backend specs and ensuring integration test coverage is sound. See discussion in the original MR !201202 (comment 2727208392)
• possible route clean up
• possible documentation updates.

Once this is merged, we can implement the task to delete the feature flag in #580736

Acceptance Criteria

The new URL Import form is the only one available to users and tested

The feature flag gates remain since import_by_url_new_page is currently enabled globally. Removing the feature flag will take place in #580736