TransportAbortHandlerIsMovingOn - removing transport abort handler

(We wish him luck in /dev/null)

Fixes workitems #3067 and #3102
`AbortHandler` could only be disposed by the transport. This means that to use a disposed `AbortHandler` you would have to either have a disposed transport (since the transport disposed `AbortHandler` instance it owned) or the transport intentionally disposed the `AbortHandler` outside `Dispose()` (none of ours did that) in which case you would have to deal with this situation yourself. Seems like setting this `AbortSent` to `true` here is trying to optimize for invalid cases. In addition it is counter intuitive and feels like a hack. If this turns out to be an issue we probably need a better solution (or name).