Removing trusted certificate from Mail in iOS 8
It looks like you might not have actually "trusted" it, but rather made "exception" for it as detailed here: https://blog.httpwatch.com/2013/12/12/five-tips-for-using-self-signed-ssl-certificates-with-ios/

If you did import the certificate, then the above website shows how to remove the "profile" that contains the certificate. To remove the exception, it looks like you need to reset all settings (General > Reset > Reset Settings). Unfortunately it deletes all your settings.

There is further discussion here:

How do you remove SSL certificate exceptions on iOS?