We had only one place left that used the `clearfix` class, in the registration form. This no longer had any effect because the captcha row is now explicitly `display: flex`, so we can remove it. We still have a `clearfix` mixin in helpers.less which is used for fieldset `<legend>` tags, and unfortunately because of how browsers treat that, we can't get rid of clearfix entirely. Testing Done: Verified that the captcha in the registration form looked correct on both desktop and mobile.