fix #1858

The channel mode +R used to both prevent joins by unregistered users,
and prevent unregistered users who happened to be joined from speaking.
This changes the behavior so that +R only prevents joins:

1. This allows users who were invited or SAJOIN'ed to speak
2. To restore the old semantics, chanops can set +RM

irc/channel.go

 	if channel.flags.HasMode(modes.Moderated) && clientModes.HighestChannelUserMode() == modes.Mode(0) {
 		return false, modes.Moderated
 	}
-	if channel.flags.HasMode(modes.RegisteredOnly) && client.Account() == "" &&
-		clientModes.HighestChannelUserMode() == modes.Mode(0) {
-		return false, modes.RegisteredOnly
-	}
 	if channel.flags.HasMode(modes.RegisteredOnlySpeak) && client.Account() == "" &&
 		clientModes.HighestChannelUserMode() == modes.Mode(0) {
 		return false, modes.RegisteredOnlySpeak