chore: remove unnecessary test helpers

[findepi]

per #13260 (comment)

[alamb]

Thanks @findepi -- this looks great. It just needs a cargo fmt and merge up from main and I think it will be good to merge 🙏

[alamb]

I tried to fix the CI but I don't have permissions to push to your fork:

(venv) andrewlamb@Andrews-MBP-2:~/Software/datafusion2$ git push -u
Enumerating objects: 13, done.
Counting objects: 100% (13/13), done.
Delta compression using up to 16 threads
Compressing objects: 100% (7/7), done.
Writing objects: 100% (7/7), 706 bytes | 706.00 KiB/s, done.
Total 7 (delta 5), reused 0 (delta 0), pack-reused 0 (from 0)
remote: Resolving deltas: 100% (5/5), completed with 5 local objects.
To github.com:findepi/datafusion.git
 ! [remote rejected]     findepi/chore-remove-unnecessary-test-helpers-f3abc1 -> findepi/chore-remove-unnecessary-test-helpers-f3abc1 (permission denied)
error: failed to push some refs to 'github.com:findepi/datafusion.git'

[findepi]

thank you @alamb !

[alamb]

I restarted the vendored code CI check as it seems it was due to a network error

[alamb]

🚀