remove outdated fixme

Author: lcnr

src/librustc_trait_selection/opaque_types.rs

@@ -1253,9 +1253,6 @@ pub fn may_define_opaque_type(
 ///
 /// Requires that trait definitions have been processed so that we can
 /// elaborate predicates and walk supertraits.
-//
-// FIXME: callers may only have a `&[Predicate]`, not a `Vec`, so that's
-// what this code should accept.
 crate fn required_region_bounds(
     tcx: TyCtxt<'tcx>,
     erased_self_ty: Ty<'tcx>,