Remove obsolete functions from OpenTelemetry.Api package (replaced with a new Extensions.Propagators package)

[dmpe]

Bug Report

As part of #3244, we have moved B3 tracing context propagation to a new Nuget package. Because of that, we have marked old public methods (in OTEL.API/Context/Propagation folder) as obsolete. Consequently, this bug is to remind us to remove these deprecated functions as part of the next major OTEL release.

We will close this issue if:

• Deprecated methods have been removed from \src\OpenTelemetry.Api\Context\Propagation, i.e. the B3Propagator has been removed.

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[github-actions]

This issue was marked stale due to lack of activity and will be closed in 7 days. Commenting will instruct the bot to automatically remove the label. This bot runs once per day.