What does this PR do?

These functions are exported and not used in our repository. They were introduced during v2 development, and have been deemed as not needed.

Chunk was exported in #2408. Its fields were unexported later, so it's an opaque type that needs to be exported for mocktracer.

Once we remove/improve the mocktracer, we'll be able to unexport it again. As it's opaque and there isn't public API using it, it won't be a breaking change for external users.

Motivation

Reduce API.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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