Remove our TracingComponent once functionality is upstreamed.

[hdevalence]

Upstream issues: iqlusioninc/abscissa#149 iqlusioninc/abscissa#154

Once these are merged, we should update our Abscissa version #89 and remove our TracingComponent, or at least the part of it that does the tracing-related parts -- we probably want to keep the HTTP endpoint until we can fold it into an RPC mechanism.

[hdevalence]

This is merged upstream (but not released), so we can remove the tracing setup parts of the TracingComponent and change it to a TracingEndpoint that just feeds HTTP requests into the built-in tracing component.

Because these changes aren't released yet, this requires a temporary git dependency on abscissa, but it's probably good to test that the tracing component is actually usable and that there's not a hole in the API before asking for a release (cc @tarcieri).

[hdevalence]

Looks like we can test this prerelease: iqlusioninc/abscissa#170

[hdevalence]

Closed by #169