Remove extra tracing span

[nachocano]

Describe the bug
It seems that we are creating an extra unreported span. #1944 improved on this a bit, but assumes that there is a *Span resident in memory, which works for InMemory, but probably not other channel implementations.
Need to use the traceparent event attribute for serialization into the CloudEvent itself.

Expected behavior
E2E tracing test checking for span should be uncommented.

Additional context
Comment from https://github.com/knative/eventing/pull/1933/files#r327255621

[nachocano]

fyi @Harwayne