Remove duplicate c-variadic.md

Author: mejrs

src/doc/unstable-book/src/library-features/c-variadic.md

@@ -93,14 +93,12 @@ pub fn check(path: &Path, features: CollectedFeatures, bad: &mut bool) {
     // Check for Unstable Book sections that don't have a corresponding unstable feature
     for feature_name in &unstable_book_lib_features_section_file_names - &unstable_lib_feature_names
     {
-        if !unstable_lang_feature_names.contains(&feature_name) {
-            tidy_error!(
-                bad,
-                "The Unstable Book has a 'library feature' section '{}' which doesn't \
+        tidy_error!(
+            bad,
+            "The Unstable Book has a 'library feature' section '{}' which doesn't \
                          correspond to an unstable library feature",
-                feature_name
-            );
-        }
+            feature_name
+        );
     }
 
     // Check for Unstable Book sections that don't have a corresponding unstable feature.