Simplification of delay during reconnect

[kfricke]

The delay method was wasting an argument, static class member and only got called from one location.

[pfalcon]

umqtt.robust.MQTTClient is intended to be configurable, extensible, and subclassable by a user.

[kfricke]

But a base class should at least show reasonable behaviour and wasteless implementation.

[pfalcon]

And that's exactly what it does.

[kfricke]

The argument i passed to the method delay is never used. I did assume that your intent was to pass a dynamic delay, which does increase over time when the connection keeps failing.
Dropping the method was just another brave brevity.