Remove support for old Boost-style message placeholder syntax

It was replaced in the v0.21 metadata syntax back in August 2023.

src/api/metadata/yaml/message.h

@@ -109,37 +109,9 @@ struct convert<loot::Message> {
         formatArgStore.push_back(sub);
       }
 
-      static const std::regex boostSyntax(
-          "%(\\d+)%", std::regex::ECMAScript | std::regex::icase);
-
       for (auto& mc : content) {
-        // Replace the old Boost.Format placeholder syntax for backward
-        // compatibility. To be removed after at least one major release of
-        // libloot to allow migration to the new syntax.
-        auto text = mc.GetText();
-        std::smatch match;
-        while (
-            std::regex_search(text.cbegin(), text.cend(), match, boostSyntax)) {
-          if (match.size() > 1) {
-            const auto n = std::stoul(match[1]);
-            if (n > 0) {
-              const auto newPlaceholder = "{" + std::to_string(n - 1) + "}";
-              text.replace(match[0].first, match[0].second, newPlaceholder);
-            } else {
-              throw RepresentationException(
-                  node.Mark(),
-                  "bad conversion: found zero-indexed placeholder using old "
-                  "syntax");
-            }
-          } else {
-            throw RepresentationException(node.Mark(),
-                                          "bad conversion: only partially "
-                                          "matched old placeholder syntax");
-          }
-        }
-
         try {
-          const auto formattedText = fmt::vformat(text, formatArgStore);
+          const auto formattedText = fmt::vformat(mc.GetText(), formatArgStore);
           mc = loot::MessageContent(formattedText, mc.GetLanguage());
         } catch (const fmt::format_error& e) {
           throw RepresentationException(

src/tests/api/internals/metadata/yaml/message_test.h

@@ -324,28 +324,6 @@ TEST_F(MessageTest,
             message.GetContent());
 }
 
-TEST_F(MessageTest, decodingFromYamlShouldAcceptPercentagePlaceholderSyntax) {
-  YAML::Node node = YAML::Load(
-      "type: say\n"
-      "content: content %1% %2% %3% %4% %5% %6% %7% %8% %9% %10% %11%\n"
-      "subs:\n"
-      "  - a\n"
-      "  - b\n"
-      "  - c\n"
-      "  - d\n"
-      "  - e\n"
-      "  - f\n"
-      "  - g\n"
-      "  - h\n"
-      "  - i\n"
-      "  - j\n"
-      "  - k");
-  Message message = node.as<Message>();
-
-  ASSERT_EQ(1, message.GetContent().size());
-  EXPECT_EQ("content a b c d e f g h i j k", message.GetContent()[0].GetText());
-}
-
 TEST_F(MessageTest, decodingFromYamlShouldThrowIfAnInvalidConditionIsGiven) {
   YAML::Node node = YAML::Load(
       "type: say\n"