[core] review and remove unused exports

[pichlermarc]

Description

Important

Do not start working on implementation yet. This work-item still needs to be approved

@opentelemetry/core is a package that holds some shared code that's used across many packages in this repository and the contrib repository. The package currently exports a large public API which we should reduce to improve maintainability.

This issue is considered done if the following steps are completed for each of the current exports in index.ts:

• if an export is deprecated, remove the export
• if an export is not used in any other package in this repository or in contrib repository, remove the export
• if an export is used in exactly one package - move its code to that package (do not export it from the package it was moved to)
• if an export is only used in tests in this repository, extract it to a new private package @opentelemetry/internal-test-utils
• if an export is only used in tests in the contrib repository, extract it to the contrib test utils package
• remove any code that becomes unused through this process

TODO: what to do with things that are better suited to other packages.
TODO: possible way to find the exports

Additional Context

• contrib repo
• list of exports