mail/ajax_get_json.php full of dead code

I was surprised how much stale code was present interface/web/mail/ajax_get_json.php

• dkim_public is passed as a parameter, but never used
• a private key is generated via shell exec, which can now be done with clean php functions.
• code to generate a new selector is never reached
• $domain is passed via a validate function, but the value comes from our database... so if its bad then we already have another problem.
• json is manually concatenated, which is more complex then needed.