[CI] Remove YAPF from `format.sh`

Once we've formatted the Python code with Black (#21316), we can remove all references to YAPF from the codebase. 

In particular, we can remove YAPF from:
* `format.sh`
* `python/requirements_linters.txt`
* `python/ray/tune/requirements-dev.txt`
* `.gitpod/Dockerfile`