Chat: remove file checker, use unix line endings

[rmackay9]

This PR makes two low risk changes:

1. removes the assistant-file-checker.py script which is unnecessary (and actually broken) since we upgraded to use ChatGPT-4o (see chat: update setup_assistant to use gpt-4o #1411)
2. changes the assistant setup files to use unix line endings instead of windows

I've tested the unix line ending change by updating the assistant in my local OpenAI account and confirming it works.