UnlabeledLoad.computeQueueLength() includes labeled jobs

The Jenkins#unlabeledLoad LoadStatistics incorrectly counts labeled jobs from computeQueueLength().

Steps to reproduce:

1. Create a job with a label 'foo'
2. Run the job and ensure it is the only job in the queue
3. Evaluating Jenkins.getInstance().unlabeledLoad.computeQueueLength() will return 1

This causes the Jenkins.unlabeledLoadProvisioner to provision slaves with null labels even though they will not be able to run the labeled jobs.