* remove unused code
I've removed the provider-specific code under registry, and unused nil
backend, and replaced a call to helper from backend/oss (the other
callers of that func are provisioners scheduled to be deprecated).
I also removed the Dockerfile, as our build process uses a different
file.
Finally I removed the examples directory, which had outdated examples
and links. There are better, actively maintained examples available.
* command: remove various unused bits
* test wasn't running
* backend: remove unused err
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