Remove reference kernels from NKI Samples

[mrkcath-aws]

Issue #, if available:

None

Description of changes:

Removing the reference directory. Instead, developers should reference NKI Library.

Testing:

Please see detailed unit test requirements in the CONTRIBUTING.md

• [ N/A ] The change is covered by numeric check using nki.baremetal
• [ N/A ] The change is covered by performance benchmark test using nki.benchmark
• [ N/A ] The change is covered by end-to-end integration test

Pull Request Checklist

• [ X ] I have filled in all the required field in the template
• [ N/A ] I have tested locally that all the tests pass
• [ X ] By submitting this pull request, I confirm that my contribution is made under the terms of the MIT-0 license.

[erickson-doug]

LGTM