Remove FIXME

This resolves the concern in favor of prohibiting multiple instances of
the attribute. This is similar to non-helper attributes as introduced in
rust-lang#88681.

Author: jhpratt

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -222,9 +222,6 @@ fn validate_default_attribute(
             "this method must only be called with a variant that has a `#[default]` attribute",
         ),
         [first, rest @ ..] => {
-            // FIXME(jhpratt) Do we want to perform this check? It doesn't exist
-            // for `#[inline]`, `#[non_exhaustive]`, and presumably others.
-
             let suggestion_text =
                 if rest.len() == 1 { "try removing this" } else { "try removing these" };