Remove an out-of-date comment.

This comment dates back to at least 2013, and is no longer relevant.
(There used to be an `allow` attribute, but that's no longer present.)

compiler/rustc_expand/src/build.rs

@@ -579,8 +579,6 @@ impl<'a> ExtCtxt<'a> {
         attrs: ast::AttrVec,
         kind: ast::ItemKind,
     ) -> P<ast::Item> {
-        // FIXME: Would be nice if our generated code didn't violate
-        // Rust coding conventions
         P(ast::Item {
             ident: name,
             attrs,