Remove out-of-date comment.

It was added in 51938c6, a commit with
a 7,720 line diff and a one line commit message. Even then the comment
was incorrect; there was a removed a `build_output_filenames` call with
a `&[]` argument in rustdoc, but the commit removed that call. In such a
large commit, it's easy for small errors to occur.

compiler/rustc_interface/src/passes.rs

@@ -602,9 +602,7 @@ fn output_filenames(tcx: TyCtxt<'_>, (): ()) -> Arc<OutputFilenames> {
     let (_, krate) = &*tcx.resolver_for_lowering(()).borrow();
     let crate_name = tcx.crate_name(LOCAL_CRATE);
 
-    // FIXME: rustdoc passes &[] instead of &krate.attrs here
     let outputs = util::build_output_filenames(&krate.attrs, sess);
-
     let output_paths =
         generated_output_paths(tcx, &outputs, sess.io.output_file.is_some(), crate_name);