message: subset lb: remove outdated TODO comment
  After refactoring to use cached predicates and deal
  with degraded/excluded hosts, there's no longer a need to
  merge the previously expensive filter calls.
diff: @@ -707,8 +707,6 @@ void SubsetLoadBalancer::HostSubsetImpl::update(const HostVector& hosts_added,
  // If we only have one locality we can avoid the first call to filter() by
  // just creating a new HostsPerLocality from the list of all hosts.
  - //
  - // TODO(rgs1): merge these two filter() calls in one loop.
  HostsPerLocalityConstSharedPtr hosts_per_locality;
  if (original_host_set_.hostsPerLocality().get().size() == 1) {
  @@ -718,9 +716,9 @@ void SubsetLoadBalancer::HostSubsetImpl::update(const HostVector& hosts_added,
  hosts_per_locality = original_host_set_.hostsPerLocality().filter({cached_predicate})[0];
  }
  - HostsPerLocalityConstSharedPtr healthy_hosts_per_locality =
  + auto healthy_hosts_per_locality =
  original_host_set_.healthyHostsPerLocality().filter({cached_predicate})[0];
  - HostsPerLocalityConstSharedPtr degraded_hosts_per_locality =
  + auto degraded_hosts_per_locality =
  original_host_set_.degradedHostsPerLocality().filter({cached_predicate})[0];
  auto excluded_hosts_per_locality =
  original_host_set_.excludedHostsPerLocality().filter({cached_predicate})[0];
  