remove unnecessary fixme

compiler/rustc_typeck/src/collect/type_of.rs

@@ -20,9 +20,6 @@
 /// This should be called using the query `tcx.opt_const_param_of`.
 #[instrument(level = "debug", skip(tcx))]
 pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<DefId> {
-    // FIXME(generic_arg_infer): allow for returning DefIds of inference of
-    // GenericArg::Infer below. This may require a change where GenericArg::Infer has some flag
-    // for const or type.
     use hir::*;
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);