Mqtt: get rid of old config

[sihui]

Help, I'm ready to give up

Please be aware of a big potential gotcha: If you define parameters in a .cfg file, but later change the key or comment out the key (to the left of the = sign), the old key and value remains in the configuration stored in the server. This can wreak havoc and lead to many otherwise inexplicable problems. Users expect, quite reasonably, that the old key no longer mentioned in the .cfg file disappears from the configuration, but it doesn’t. This is particularly irksome when you use a 1.x bindi…