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.

Verified that the captcha in the registration form looked correct on

both desktop and mobile.