Remove Lightstep/Stackdriver exporter from this repo

[reyang]

Following open-telemetry/opentelemetry-specification#277 (comment), we need to move vendor specific exporters away from this repo.
Need to make sure the exporter is registered under https://opentelemetry.io/registry/?s=net.

This needs to go somewhere else: https://github.com/open-telemetry/opentelemetry-dotnet/tree/master/src/OpenTelemetry.Exporter.LightStep.

The end result would be having only these exporters in the repo:

• Console
• Jaeger
• Noop
• OpenTelemetryProtocol
• Prometheus
• Zipkin

[bruno-garcia]

We've discussed this on Gitter and the following proposal arrived:

1. Create a repo: https://github.com/open-telemetry/opentelemetry-dotnet-contrib which @MikeGoldsmith is raising an issue for.
2. Move all vendor-specific exporters to it.
3. Depend on OpenTelemetry packages via NuGet dependency.

[MikeGoldsmith]

PR to move StackDriver to contrib project: open-telemetry/opentelemetry-dotnet-contrib#3