Remove unused cold_start experiment (#87470)
- this `--cold_start` experiment didn't end up being used
- there is a new `--cold_start_latency` flag that is used
- this experiment was only hooked up for nvfuser anyway
cc @jansel @lezcano @fdrocha @mlazos @soumith @voznesenskym @yanboliang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87470
Approved by: https://github.com/anijain2305
