Clear Axons and Prometheus on uid deregistration

[gztensor]

1. This function should remove the currently being deregistered hotkey from Axons and Prometheus maps:
   

   subtensor/pallets/subtensor/src/subnets/uids.rs

   Line 23 in 8f33f8c

   pub fn clear_neuron(netuid: NetUid, neuron_uid: u16) {

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

[l0r1s]

@gztensor I added the migration here #2212, it does remove all Axon/Cert/Prometheus that have hotkey not present in the UIDs of the subnet. Does it looks good to you?

Also, the clear_neuron is only responsible for resetting the neuron value, not deleting them so I think they should be kept?