Removed deprecated functions, deprecate testing.open_dataset

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Removes the deprecated sfcwind_2_uas_vas and uas_vas_2_sfcwind converter functions.
• Adds a deprecation notice to xclim.testing.open_dataset to be removed in a future version

Does this PR introduce a breaking change?

Yes. Functions that were previously deprecated have been removed. Suggestions have been made to use the nimbus class to manage fetching testing data.