Suricata

Although our MQTT app-layer parser is mostly in rust, some of the parser registration code still resides in app-layer-mqtt.c

It is possible to move that code to Rust, therefore getting rid of the related c files.

This task encompasses:
- moving the existing code from app-layer-mqtt.c to the rust mqtt parser file
- removing now unnecessary app-layer-mqtt.* files
- checking if there's any left over lines of code that must be adjusted.