Remove Agent Email

Remove the agent's email association.

Uniform response whether or not one was set — "you had nothing to remove" is information about this account, not about an address, but keeping it uniform costs nothing and avoids a needless distinction.

Clearing does NOT decrement the fingerprint's distinct_holders. That is the point of the cycle cap: an address that has moved through accounts has moved through them, and letting a delete roll the counter back would make create-delete-recreate free again.