feat(3p/nix): remove External values feature

External values are only useful when using the plugin framework, which we are not interested in carrying forward.

Reverts commit 320659b0cd

Change-Id: Ib4929c349bbb33f16224fc674e94c7b7d5953c6a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1505
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

third_party/nix/src/libexpr/primops.cc

@@ -193,9 +193,6 @@ static void prim_typeOf(EvalState& state, const Pos& pos, Value** args,
     case tPrimOpApp:
       t = "lambda";
       break;
-    case tExternal:
-      t = args[0]->external->typeOf();
-      break;
     case tFloat:
       t = "float";
       break;