Less verbose debugging (also don't log private keys)

Magicsig private keys were logged. That's probably not a good thing.
MagicEnvelope full XML entries no longer spam the log either.

plugins/OStatus/lib/magicenvelope.php

@@ -188,7 +188,6 @@ class MagicEnvelope
         $xs->elementEnd('me:env');
 
         $string =  $xs->getString();
-        common_debug('MagicEnvelope XML: ' . $string);
         return $string;
     }