Remove 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.Stackdriver.

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

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

[MikeGoldsmith]

Requested contrib project for any exporters that are not reassigned to owning organisations: open-telemetry/community#340

We can also push other non-core code here. Utilities, helpers, etc.

[reyang]

#646