NO-ISSUE

Problem

Refactoring

Solution

Simplify dbt_utils.group_by implementation.

Checklist

• [ x] This code is associated with an issue which has been triaged and accepted for development.
• [x ] I have read the contributing guide and understand what's expected of me
• [x ] I have run this code in development and it appears to resolve the stated issue
• [x ] This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)