delete helper/fetch-channel

This script was obsoleted by switching to flakes.

helper/fetch-channel

@@ -1,6 +0,0 @@
-#!/usr/bin/env bash
-
-rev=$1
-sha256=$(nix-prefetch-url --unpack https://github.com/nixos/nixpkgs/archive/$rev.tar.gz)
-echo "rev = \"$rev\";"
-echo "sha256 = \"$sha256\";"