is it possible to remove a registered key from the snap store?
it’s not possible to remove keys at the moment.
However, note/remember that this is public/private key cryptography. Only the public key is ever uploaded to the store. The private key always remains in the developer’s computer. If you delete that one for a keypair you no longer want to use, the private key store-side will become effectively orphaned/useless since it’s basically impossible to ever generate a private key that will match it, which should be as good/secure as deleting it.
Deletion for administrative/housekeeping purposes is not possible, however - apologies for that.