1. Remove the test/test_mqtt_broker and use emqx-ct-helpers -> Done!
    - Enhance all test case

2. Remove the mqtt adaptor
3. Remove the emqx_coap_pubsub_topics.erl


### Problems

1. The coap-client of libcoap does not support Fragment DTLS handshake frame
    * So, the connection will be established failed, if the 'Server Hello' frame is too big
    * Why is the 'Server Hello' too big when enable the 'coap.dtls.cacertfile' option?
2.