Delete never-implemented stub definition of AsyncDataProvider

[PhilMiller]

The class AsyncDataProvider is abstract, has no implementation, no tests, and no putative uses. Per git log -SAsyncDataProvider, is has never been referenced other than in the commit that introduced it.

Removals

• Delete include/forcing/AsyncDataProvider.hpp

Testing

1. CI

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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