Remove "let config" warning

[kubouch]

Description

If you have a config variable defined at some point after reading config files, Nushell would print

warning: use `$env.config = ...` instead of `let config = ...`

I think it's long enough since we've used $env.config that we can remove this. Furthermore, it should be printed during let parsing because you can end up with a config constant after importing a config module (that was my case). The warning thus can be misleading.

User-Facing Changes

Tests + Formatting

After Submitting

[fdncred]

Agreed.