Consider not cancelling consumers on channel closure

[kjnilsson]

There is no value in first cancelling all consumers then immediately closing the channel as the consumers will be correctly cleaned up when the channel is closed by RabbitMQ. Consider removing the code around this line: https://github.com/pika/pika/blob/master/pika/channel.py#L544

In RabbitMQ 3.8 we will ship a feature that may require that basic.cancel is not sent before closing the channel for certain guarantees to hold.