experimental-features.hh: Don't include json-utils.hh

This caused nlohmann/json.hpp to leak into a lot of compilation units,
which is slow (when not using precompiled headers).

Cuts build time from 46m24s to 42m5s (real time with -j24: 2m42s to
2m24s).

src/libutil/experimental-features.hh

@@ -2,9 +2,10 @@
 ///@file
 
 #include "error.hh"
-#include "json-utils.hh"
 #include "types.hh"
 
+#include <nlohmann/json_fwd.hpp>
+
 namespace nix {
 
 /**
@@ -98,10 +99,4 @@ public:
 void to_json(nlohmann::json &, const ExperimentalFeature &);
 void from_json(const nlohmann::json &, ExperimentalFeature &);
 
-/**
- * It is always rendered as a string
- */
-template<>
-struct json_avoids_null<ExperimentalFeature> : std::true_type {};
-
 }