How can I prevent the display of CAPTCHA for other forms except the login form?
The message "CAPTCHA: no challenge enabled" indicates, that the form in question is not protected by a captcha.

This message is shown to site administrators and users with the "Administer CAPTCHA settings" permission only. It is intended to ease administering captchas on the form. You could, e.g. click the message to open a link that allows you to enable a captcha for this form.

If you don't require these administrative links, you can disable them within the captcha module settings. In your administrative back-end navigate to Configuration > People > CAPTCHA.

Locate the option "Add CAPTCHA administration links to forms", uncheck it and save the settings. The administration links should be gone.