Simplify B3 options

[mwear]

We can streamline b3 propagation based on this spec issue: open-telemetry/opentelemetry-specification#1562 (comment).

Right now we have three b3 propagator variants.

• We have a B3Propagator that extracts b3 single and b3 multi, and injects b3 single, but is configurable to inject b3 multi; this is technically spec compliant
• We have a B3SinglePropagator that exclusively injects and extracts b3 single
• We have a B3MultiPropagator that exclusively injects and extracts b3 multi

Based on the discussion on the spec PR, there is not really a use case where we'd want to only extract one b3 format; we always want to extract both single and multi. I think we can remove the B3Propagator and update the B3SinglePropagator and B3MultiPropagator to extract both formats.

[mwear]

After taking a second look our implementation, I like the B3Propagator as the public interface for b3. I think we should probably treat the B3SinglePropagator and B3MultiPropagator as implementation details leave them unexported.