fix CHATHISTORY 005 token name

Unclear where we got draft/CHATHISTORY from, it looks like the merged drafts
have always used unprefixed CHATHISTORY as the token name.

irc/config.go

 	isupport.Add("CHANLIMIT", fmt.Sprintf("%s:%d", chanTypes, config.Channels.MaxChannelsPerClient))
 	isupport.Add("CHANMODES", chanmodesToken)
 	if config.History.Enabled && config.History.ChathistoryMax > 0 {
-		isupport.Add("draft/CHATHISTORY", strconv.Itoa(config.History.ChathistoryMax))
+		isupport.Add("CHATHISTORY", strconv.Itoa(config.History.ChathistoryMax))
 	}
 	isupport.Add("CHANNELLEN", strconv.Itoa(config.Limits.ChannelLen))
 	isupport.Add("CHANTYPES", chanTypes)