Implement deleting a key pair by deleting the key pair

Weird, isn't it? If you delete a public key (which may or may be not a part of a key pair) you have to invalidate all signatures from this key. Until now after deleting a secret key all secret keys were simply reloaded. This was wrong in multiple different ways as it did not do everything expected. Also it is inefficient to reload multiple keys.

Now that we have notifications between public keys and signature it's much cleaner: delete the key node if it is gone from keyring and that's it.

This one should be really gone now. There is no chance that the key pair is still there if the key is deleted. At least I hope so ;)