Remove assertion about self handle after connection disposal

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>

When a connection is disposed, the self handle is not always reset. But there is no good reason to have the sub-class assert about a parent-class state. Also, the connection object is really disposed and not re-used, so this clean-up detail is really not important.