Removed use of cache in JobQueueFederated pop() method

* This does not really buy anything for a number of reason and
  can cause bugs. For example, if the aggregator is notified due
  to undelayed jobs but this cache is not updated, then the runners
  will not see the jobs and will de-list aggregator entry.

bug: 68506
Change-Id: I1eacca108df59fff9c0fcc846aafaf1616cf5895