fix-protypes: Fix `mqtt_agent_task.h` prototypes

The only public function declared in `mqtt_agent_task.c` is
`vStartMqttAgentTask` as other functions were extracted to separate
files in previous work.
The prototypes for these other functions were left behind in
`mqtt_agent_task.h`. This commit removes them.

Also change `ota_agent_task.c` to include `events.h`, which is where
these function prototypes were placed in previous work.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>