Remove super old comment on function that parses items

This comment was added more than 5 years ago in ab03c1e422. As far as
anyone reading this comment today needs to know, the function has never
parsed items from inside an extern crate.

src/libsyntax/parse/parser.rs

@@ -6842,8 +6842,6 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse one of the items allowed by the flags.
-    /// NB: this function no longer parses the items inside an
-    /// extern crate.
     fn parse_item_implementation(
         &mut self,
         attrs: Vec<Attribute>,