[Service Bus] Remove cancellation check (Azure#34537)

The focus of these changes is to remove a check of the cancellation
token during message scheduling, as the impact of doing so for each message
is higher than the benefit.

Author: jsquire

sdk/servicebus/Azure.Messaging.ServiceBus/src/Amqp/AmqpSender.cs

@@ -457,7 +457,6 @@ internal async Task<IReadOnlyList<long>> ScheduleMessageInternalAsync(
                     }
 
                     entries.Add(entry);
-                    cancellationToken.ThrowIfCancellationRequested<TaskCanceledException>();
                 }
 
                 request.Map[ManagementConstants.Properties.Messages] = entries;