Removes leftover calls to event_handler

The possibility to add a callback as custom handler was removed from
the client in favor of esp_event. These removes the older alternative
that wasn't possible to use.

lib/include/mqtt_client_priv.h

@@ -61,7 +61,6 @@ typedef struct mqtt_state {
 } mqtt_state_t;
 
 typedef struct {
-    mqtt_event_callback_t event_handle;
     esp_event_loop_handle_t event_loop_handle;
     int task_stack;
     int task_prio;