Remove nix channel placement

[Hoverbear]

Description

We don't need to do this at all!

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

Seems fine to me. But depending on the 2.14 fallout, we might need to resurrect this for legacy CLI use cases.

[Hoverbear]

Hmmm, should we block this until we find out?

[cole-h]

I think it would be fine as-is, but if (when) we switch to 2.14 as the default for this installer, we'll probably want to make sure it's fine this is removed.

[RaitoBezarius]

Does this mean that this installer cannot install channels at all?

[Hoverbear]

The installer has no options for channels, no.