Remove old todo: Don't hard code activation link.

[david.binda]

I've stumbled upon this TODO line in wp-includes/ms-functins.php file:

// TODO: Don't hard code activation link.

It originates in ​r 543 of the mu repository. By that time, the activation link ​was really hardcoded:

$message = sprintf(__("To activate your blog, please click the following link:\n\n%s\n\nAfter you activate, you will receive *another email* with your login.\n\nAfter you activate, you can visit your blog here:\n\n%s\n\nAnd you can login on the home page:\n\nhttp://wordpress.com/"), 
    "http://wordpress.com/activate/$key", "http://$domain");
    // TODO: Don't hard code activation link.

The current message is being composed using the $activate_url, which has been introduced in ​r 550 of the mu repository, I believe that the TODO comment is no longer needed.

[SergeyBiryukov]

As far as I can see, $activate_url was introduced in ​mu:585.

The hardcoding, however, was addressed earlier in ​mu:547, so the comment is indeed no longer relevant.

[SergeyBiryukov]

In 46367:

Docs: Remove obsolete comment in wpmu_signup_blog_notification() and wpmu_signup_user_notification().

Props david.binda.
Fixes #48185.