Function esp_mqtt_client_unregister_event 

pub unsafe extern "C" fn esp_mqtt_client_unregister_event(
    client: *mut esp_mqtt_client,
    event: i32,
    event_handler: Option<unsafe extern "C" fn(*mut c_void, *const u8, i32, *mut c_void)>,
) -> i32

@brief Unregisters mqtt event

@param client mqtt client handle @param event event ID @param event_handler handler to unregister

@return ESP_ERR_NO_MEM if failed to allocate ESP_ERR_INVALID_ARG on invalid event ID ESP_OK on success