Retained MQTT messages are not cleared when MQTT is disabled, should they? #284

Describe the bug
When MQTT is enabled there are some retained messages created such as motion_image, sound_detection etc.
When MQTT is disabled these are left in the broker. Would it make sense (is it good practice?) for these to be cleared from the broker?

To Reproduce
Steps to reproduce the behaviour:

1. Enable MQTT
2. Allow some retained messages to be sent
3. Disable MQTT
4. Retained messages are retained

Expected behaviour
Retained messages are cleared.

Set Up Details (please complete the following information):

• Firmware Version: 0.4.0
• Device: Yi Outdoor
• Model: 9CUS