Remove needless attrs reference

It's a little tricky to follow, but if you trace the code paths for
inline modules and for external modules, this value is never actually
used. This makes two functions more similar so that they can be deduped
in a later commit.

Author: camsteffen

src/modules.rs

@@ -212,7 +212,7 @@ impl<'ast, 'sess, 'c> ModResolver<'ast, 'sess> {
                         span,
                         Some(Cow::Borrowed(sub_mod_kind)),
                         Cow::Owned(ThinVec::new()),
-                        Cow::Borrowed(&item.attrs),
+                        Cow::Owned(ast::AttrVec::new()),
                     ),
                 )?;
             }