Besides automation, running
rm -rf against directories is my second to none thing to do as a DevOps engineer. And this is exactly how you could easily remove a Git repository.
Step 1. Go to your current Git project directory.
Step 2. Run the sacred command:
rm -rf .git
Step 3. Verify.
fatal: not a git repository (or any of the parent directories): .git
To remove a remote Git repo which is usually hosted on GitHub or GitLab you must follow the official platform-specific docs.