If you delete an SQS queue, all the messages in the queue will be deleted as well.
To delete an SQS queue from AWS CLI, use the below commands. First obtain the url of the queue as below:
aws sqs get-queue-url --queue-name queuename
Substitute the above obtained url in below command to delete the queue.
aws sqs delete-queue --queue-url <url-of-the-queue>