Cleanup: remove scary warning about reading Library data-block

From what I can tell, this was fixed a long time ago already in 42057481fbb0c7ed3.
See the change from `(Library *)(bheadlib+1)` to `read_struct(fd, bheadlib, "Library")`.

The warning was added 5 months before that in f1a217c5f9f05045.

Pull Request: https://projects.blender.org/blender/blender/pulls/120272

source/blender/makesdna/DNA_ID.h

@@ -540,8 +540,7 @@ typedef struct Library_Runtime {
 } Library_Runtime;
 
 /**
- * For each library file used, a Library struct is added to Main
- * WARNING: `readfile.cc`, expand_doit() reads this struct without DNA check!
+ * For each library file used, a Library struct is added to Main.
  */
 typedef struct Library {
   ID id;