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/value-to-json.cc

@@ -73,10 +73,6 @@ void printValueAsJSON(EvalState& state, bool strict, Value& v,
       break;
     }
 
-    case tExternal:
-      v.external->printValueAsJSON(state, strict, out, context);
-      break;
-
     case tFloat:
       out.write(v.fpoint);
       break;
@@ -92,10 +88,4 @@ void printValueAsJSON(EvalState& state, bool strict, Value& v,
   printValueAsJSON(state, strict, v, out, context);
 }
 
-void ExternalValueBase::printValueAsJSON(EvalState& state, bool strict,
-                                         JSONPlaceholder& out,
-                                         PathSet& context) const {
-  throw TypeError(format("cannot convert %1% to JSON") % showType());
-}
-
 }  // namespace nix