Remove todo in facet_type.cpp after exploration (#5950)

We looked at it in
https://github.com/carbon-language/carbon-lang/pull/5947 and decided not
to proceed with it.

toolchain/check/facet_type.cpp

@@ -627,8 +627,6 @@ auto MakePeriodSelfFacetValue(Context& context, SemIR::TypeId self_type_id)
                    // `None` because there is no equivalent non-symbolic value.
                    .value_id = SemIR::InstId::None,
                }));
-  // TODO: LookupOrAddName should (optionally?) take a callback to run and
-  // construct the `inst_id` only if it's not found by lookup.
   auto existing =
       context.scope_stack().LookupOrAddName(SemIR::NameId::PeriodSelf, inst_id);
   // Shouldn't have any names in newly created scope.