Remove TChannel from agent->collector path

[yurishkuro]

@pavolloffay I created a new internal build that uses a local copy of the /tchannel packages you created (#2105), and deployed it today. All metrics look good. I think we can start removing the code.

[pavolloffay]

Do you want to submit a PR? If no I can do that.

[yurishkuro]

Go for it, I won't be able to get to it soon.

One thing I wanted to watch out was making sure we don't leave other dead code around. For example, pkg/discovery may be only used from tchannel.

[pavolloffay]

I am wondering whether full removal would break OTEL collector they are exposing tchannel exporter.

Afther quick inspection it looks that they are not using tchannel from agent or the new tchannel package cc) @objectiser
https://github.com/open-telemetry/opentelemetry-collector/pull/636/files#diff-0e6ca11168918e81d990fd9005b9671cL45