eth: unregister peer only when handler exits (#22908)

This removes the error log message that says 

    Ethereum peer removal failed ... err="peer not registered"

The error happened because removePeer was called multiple
times: once to disconnect the peer, and another time when the
handler exited. With this change, removePeer now has the sole
purpose of disconnecting the peer. Unregistering happens exactly
once, when the handler exits.