Remove height() from IbcEvent

[plafer]

IbcEvent::height() doesn't belong in the IbcEvent. This field was only tacked on IbcEvent for relayer purposes. We should instead create a IbcEventWithHeight struct in the relayer that tracks extra height data.

struct IbcEventWithHeight {
    event: IbcEvent,
    height: Height,
}

Contributes towards informalsystems/ibc-rs#11.