Make the client remove votes for elected custodians

Right now when a custodian has been elected, the clients continue to vote for him unless the user manually removes the vote.

Since they can't be elected twice, it unnecessarily fills the blockchain.

The clients should automatically remove votes for elected custodians. But it could be a problem if there's a reorganization that cancels a grant.

So instead of removing the vote, maybe the client could just skip votes for elected custodians when it generates the vote transaction.

After this is solved we could enforce this behavior at protocol level by rejecting blocks voting for elected custodians.