pkgs/splice: drop reference to past code

References to previous states of the code generally only belong in
commit messages.
A reminder or warning to future readers is not needed here.

pkgs/top-level/splice.nix

@@ -32,8 +32,6 @@ let
         # The same pkgs sets one probably intends
         // inputs.buildHost
         // inputs.hostTarget;
-      # perf: mapAttrs defers merge calls until a key is selected, avoiding
-      # ~60k eager closures that listToAttrs+map would create.
       merge =
         name: defaultValue:
         let