def _remove(client)
      return unless @clients.include? client
      channel = @clients.delete(client)
      debug { "Removing #{client} from #{channel}." }
      @channels[channel] -= 1
      @channels.delete(channel) if @channels[channel].zero?
    end
def _remove(client)
      return unless @clients.include? client
      channel = @clients.delete(client)
      debug { "Removing #{client} from #{channel}." }
      @channels[channel] -= 1
      @channels.delete(channel) if @channels[channel].zero?
    end
Removes client from +@clients+, and decrements channel's client count. If
 channel has no clients, it is removed.
 @param [String] client ID