Change selectors.New to not return an error

[jhrozek]

Summary

This is just a small refactoring that makes future patches easier to apply.

The function was only ever returning nil anyway and it would make using
it in minder easier as well

Change Type

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

just make to be honest

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 54.306% (-0.005%) from 54.311%
when pulling a8b37e9 on jhrozek:selectors_no_err_constructor
into 211f5f3 on stacklok:main.