docs(nix): remove unobserved let rule

docs/nix.md

@@ -154,31 +154,6 @@ in
 lib.genAttrs hostNames mkHost
 ```
 
-### Bind shared expressions once
-
-Use a `let` binding when sibling expressions depend on the same derived value.
-
-Avoid:
-
-```nix
-{
-  hostNames = lib.attrNames hosts;
-  hostCount = lib.length (lib.attrNames hosts);
-}
-```
-
-Use:
-
-```nix
-let
-  hostNames = lib.attrNames hosts;
-in
-{
-  inherit hostNames;
-  hostCount = lib.length hostNames;
-}
-```
-
 ### Keep bindings in the smallest scope
 
 Place a `let` binding inside the expression that consumes it.