Delete defunct DataSelector classes

[PhilMiller]

The uses of these classes were deleted in commit acea24d in 2022. Clear them out so they don't distract from on-going work

Removals

• Classes {BMI,CSV,NetCDF}DataSelector that were all empty subclasses of CatchmentAggrDataSelector

Changes

• {CSV,NetCDF}DataProvider tests modified to use base class instead of specific derived classes

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)
• Reviewers requested with the Reviewers tool ➡️