remove duplicate stubs

src/doc/unstable-book/src/library-features/proc-macro.md

@@ -1,7 +0,0 @@
-# `proc_macro`
-
-The tracking issue for this feature is: [#38356]
-
-[#38356]: https://github.com/rust-lang/rust/issues/38356
-
-------------------------

src/tools/tidy/src/unstable_book.rs

@@ -87,7 +87,9 @@ pub fn check(path: &path::Path, bad: &mut bool) {
     // Library features
 
     let lang_features = collect_lang_features(path);
-    let lib_features = collect_lib_features(path);
+    let lib_features = collect_lib_features(path).iter().filter(|(name, _) {
+        !lang_features.contains_key(name)
+    }).collect();
 
     let unstable_lib_feature_names = collect_unstable_feature_names(&lib_features);
     let unstable_book_lib_features_section_file_names =

src/tools/unstable-book-gen/src/main.rs

@@ -129,7 +129,11 @@ fn main() {
     let dest_path = Path::new(&dest_path_str).join("src");
 
     let lang_features = collect_lang_features(src_path);
-    let lib_features = collect_lib_features(src_path);
+    let lib_features = collect_lib_features(src_path)
+                                            .iter()
+                                            .filter(|(name, _) {
+                                                !lang_features.contains_key(name)
+                                            }).collect();
 
     let doc_src_path = src_path.join(PATH_STR);