This commit is a breaking change in RabbitMQ 4.0.
## What?
Remove mqtt.default_user and mqtt.default_pass
Instead, rabbit.anonymous_login_user and rabbit.anonymous_login_pass
should be used.
## Why?
RabbitMQ 4.0 simplifies anonymous logins.
There should be a single configuration place
```
rabbit.anonymous_login_user
rabbit.anonymous_login_pass
```
that is used for anonymous logins for any protocol.
Anonymous login is orthogonal to the protocol the client uses.
Hence, there should be a single configuration place which can then be
used for MQTT, AMQP 1.0, AMQP 0.9.1, and RabbitMQ Stream protocol.
This will also simplify switching to SASL for MQTT 5.0 in the future.
David Ansari
kjnilsson