libexpr: Delete EvalState::mkSingleDerivedPathString dead function with Value& out param

Part of #1136, I believe this cleans up the final instance of `Value&`
used as an out parameter.

Change-Id: I40153b1f11d5ca9af06cd427f22896d06a6a6964

lix/libexpr/eval.cc

@@ -908,19 +908,6 @@ std::string EvalState::mkSingleDerivedPathStringRaw(
 }
 
 
-void EvalState::mkSingleDerivedPathString(
-    const SingleDerivedPath & p,
-    Value & v)
-{
-    v = {
-        NewValueAs::string,
-        mkSingleDerivedPathStringRaw(p),
-        NixStringContext{
-            std::visit([](auto && v) -> NixStringContextElem { return v; }, p),
-        }
-    };
-}
-
 struct CachedEvalFile
 {
     Value result;

lix/libexpr/eval.hh

@@ -817,15 +817,6 @@ public:
      */
     Value mkOutputString(const SingleDerivedPath::Built & b, const StorePath & staticOutputPath);
 
-    /**
-     * Create a string representing a `SingleDerivedPath`.
-     *
-     * A combination of `mkStorePathString` and `mkOutputString`.
-     */
-    void mkSingleDerivedPathString(
-        const SingleDerivedPath & p,
-        Value & v);
-
     Value updateAttrs(const Value & v1, const Value & v2);
     Value concatLists(std::span<Value> lists, const PosIdx pos, std::string_view errorCtx);