HI there,
Our SSL certificate is removed the moment you cancel the mapping upgrade with us, but your browser doesn't know that.
If a site has an SSL certificate, it places an instruction in your browser in what's called a HSTS header to always load the domain over https. That header in your browser is what's causing it to try and still load the domain over https, even though the domain no longer has an SSL certificate.
That header cannot be removed manually like browser cookies can, but it should expire in a day or two at which point you'll be able to load the domain over an http connection. For now, if your browser has an option to add an exception for the domain, go ahead and do that. Otherwise you'll have to use a different browser for the next couple of days.
Visitors to your site shouldn't see that warning, unless they recently visited your site while the SSL cert was still valid, in which case they'll also need to wait for the HSTS headers in their browsers to expire.