Remove harness.go from internal/internaltest

[MrAlias]

This file contains unused types. It is not needed.

This is a generated file, so internal/gen.go will need to be updated.

This file contains the internaltest package documentation. The harness.go.tmpl file needs to be updated. The package documentation from that file needs to be moved to a dedicated doc.go.tmpl file. That new template needs to then be generated in internal/internaltest and any other spot in the project where internaltest is used:

• sdk/internal/internaltest
• exporters/zipkin/internal/internaltest

[stooj]

I'm working on this.