aliases: remove hacky workaround for warnings on strings

This causes confusing eval errors when not using these values (here:
`system`) in a string interpolation context.

The change will potentially be noisy for nix search. If we want a
deprecation period for this attribute instead of just throwing
immediately, we will have to accept this, at least temporarily.

pkgs/top-level/aliases.nix

@@ -230,11 +230,6 @@ let
       arg: lib.warn msg (v arg)
     else if lib.isList v then
       map (lib.warn msg) v
-    else if lib.isString v then
-      # Unlike the other cases, this changes the type of the value and
-      # is therefore a breaking change for some code, but it’s the best
-      # we can do.
-      { __toString = lib.warn msg (lib.const v); }
     else
       # Can’t do better than this, and a `throw` would be more
       # disruptive for users…