Remove refernece to manually splitting path by `char esep` in setup/cookbook.md

[Context](https://www.nushell.sh/cookbook/setup.html#configure-your-path-and-other-environment-variables)

I think we should avoid suggesting this directly as it's probably more confusing than it needs to be with path's ENV_CONVERSIONS being default now. We should switch this to link to https://www.nushell.sh/book/configuration.html#path-configuration probably