You'll have to edit your mysensors.json file (or the name you have at configuration.yaml), and remove the node from that file, with homeassistant stopped before. Normally the format is <node number>: { <lot of data> }.
This is one of my nodes from HA file.

"1": {"sensor_id": 1, "type": 17, "sketch_version": "1.0", "battery_level": 52, "protocol_version": "2.0.0-beta", "sketch_name": "Contact1", "children": {"0": {"id": 0, "type": 0, "values": {"16": "0"}}}}

Just delete that part, and start HA again.

On your sketch... (from https://www.mysensors.org/download/sensor_api_15)

 gw.sendBatteryLevel(batteryLevel);

But you'll have to measure batteryLevel before someway ;)