Actually remove the "flakes" experimental feature

To avoid annoying warnings, this is now a "stabilized" feature.

src/libutil/config.cc

@@ -341,8 +341,10 @@ template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeatur
 {
     std::set<ExperimentalFeature> res;
     for (auto & s : tokenizeString<StringSet>(str)) {
-        if (auto thisXpFeature = parseExperimentalFeature(s); thisXpFeature)
+        if (auto thisXpFeature = parseExperimentalFeature(s))
             res.insert(thisXpFeature.value());
+        else if (stabilizedFeatures.count(s))
+            debug("experimental feature '%s' is now stable", s);
         else
             warn("unknown experimental feature '%s'", s);
     }

src/libutil/experimental-features.cc

@@ -70,16 +70,6 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         )",
         .trackingUrl = "https://github.com/NixOS/nix/milestone/42",
     },
-    {
-        .tag = Xp::Flakes,
-        .name = "flakes",
-        .description = R"(
-            *Enabled for Determinate Nix Installer users since 2.19*
-
-            See the manual entry for [`nix flake`](@docroot@/command-ref/new-cli/nix3-flake.md) for details.
-        )",
-        .trackingUrl = "https://github.com/NixOS/nix/milestone/27",
-    },
     {
         .tag = Xp::FetchTree,
         .name = "fetch-tree",
@@ -302,12 +292,18 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
 static_assert(
     []() constexpr {
         for (auto [index, feature] : enumerate(xpFeatureDetails))
-            if (index != (size_t)feature.tag)
+            if (index != (size_t) feature.tag)
                 return false;
         return true;
     }(),
     "array order does not match enum tag order");
 
+/**
+ * A set of previously experimental features that are now considered
+ * stable. We don't warn if users have these in `experimental-features`.
+ */
+std::set<std::string> stabilizedFeatures{"flakes"};
+
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)
 {
     using ReverseXpMap = std::map<std::string_view, ExperimentalFeature>;

src/libutil/experimental-features.hh

@@ -19,7 +19,6 @@ enum struct ExperimentalFeature
 {
     CaDerivations,
     ImpureDerivations,
-    Flakes,
     FetchTree,
     NixCommand,
     GitHashing,
@@ -38,6 +37,8 @@ enum struct ExperimentalFeature
     VerifiedFetches,
 };
 
+extern std::set<std::string> stabilizedFeatures;
+
 /**
  * Just because writing `ExperimentalFeature::CaDerivations` is way too long
  */