[spec] Do not export NOOP implementations

[dyladan]

The NOOP implementations are currently exported. Particularly the NOOP tracer is used to create NOOP spans. After #51, this will no longer be required. Because the specification does not say to export NOOP implementations, we should remove their exports before 1.0

[Flarna]

As far as I know this is already the case.

Refs: open-telemetry/opentelemetry-js#1899 (that time it was still named NoopSpan)

[dyladan]

We do export NonRecordingSpan. The spec requires us not to. After #51 it will be possible to create one using that method so we will no longer have to export the constructor directly.

edit: it seems you were right we do not. but we do export the noop tracer which the API does not say to export. IMO we should limit exports that are not spec-required if at all possible.

[dyladan]

@Flarna updated description

[dyladan]

Done in #46