Clear Axons and Prometheus on uid deregistration

1. This function should remove the currently being deregistered hotkey from Axons and Prometheus maps:
https://github.com/opentensor/subtensor/blob/8f33f8cbf6b958b9ec215424a50d96cd2fc5e5ae/pallets/subtensor/src/subnets/uids.rs#L23

2. Write a one-time migration that will remove Axons and Prometheus entries for all netuids that have hotkey not currently registered in this subnet