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-xml.cc

@@ -163,11 +163,6 @@ static void printValueAsXML(EvalState& state, bool strict, bool location,
       break;
     }
 
-    case tExternal:
-      v.external->printValueAsXML(state, strict, location, doc, context,
-                                  drvsSeen);
-      break;
-
     case tFloat:
       doc.writeEmptyElement(
           "float", singletonAttrs("value", (format("%1%") % v.fpoint).str()));
@@ -178,13 +173,6 @@ static void printValueAsXML(EvalState& state, bool strict, bool location,
   }
 }
 
-void ExternalValueBase::printValueAsXML(EvalState& state, bool strict,
-                                        bool location, XMLWriter& doc,
-                                        PathSet& context,
-                                        PathSet& drvsSeen) const {
-  doc.writeEmptyElement("unevaluated");
-}
-
 void printValueAsXML(EvalState& state, bool strict, bool location, Value& v,
                      std::ostream& out, PathSet& context) {
   XMLWriter doc(true, out);