const_generics_defaults: don't use todo

So that at least it won't ICE for users whether or not they enable the gate. For developers the FIXMEs are enough.

compiler/rustc_hir_pretty/src/lib.rs

@@ -2210,7 +2210,6 @@ impl<'a> State<'a> {
                 self.print_type(ty);
                 if let Some(ref _default) = default {
                     // FIXME(const_generics_defaults): print the `default` value here
-                    todo!();
                 }
             }
         }