rustdoc: Don't bother reporting the type of arguments

This is already displayed in the function signature. Simpler this way.

src/rustdoc/extract.rs

@@ -161,8 +161,7 @@ fn argdocs_from_args(args: [ast::arg]) -> [doc::argdoc] {
 fn argdoc_from_arg(arg: ast::arg) -> doc::argdoc {
     {
         name: arg.ident,
-        desc: none,
-        ty: none
+        desc: none
     }
 }