Remove uncessary add pubkey
Closes #1998
When signer started , it get all the vault pubkeys that the local node has ever been part of , and add it to PubKeyManager, which is not necessary.
- a node can belongs to one asgard at a time , so only the most latest membership is relevant, which PubKeyManager will download from thornode when it start
- PubKeyManager will refresh the pub keys every 100 blocks ,and prune the old ones , so the old keys will be removed anyway
The other two places to call PubKeyManager.GetPubKeys() , but didn't use it , so it is obvious