Remove GetDescriptor method from SDK

[MrAlias]

This method was added to accommodate local testing, but is an exported method accessible outside of the package. Returning access to the underlying descriptor, an object we hope to treat as immutable, is not something we want to do.

The internal testing (AFAIK the stress test is the only test that relies on this) need to be integrated into the package so they can access the descriptor directly or use an unexported method.

Originating from comment posted by @MrAlias in https://github.com/_render_node/MDIzOlB1bGxSZXF1ZXN0UmV2aWV3VGhyZWFkMjQ0NjUzMjkwOnYy/pull_request_review_threads/discussion