Clean up usage of global checkpoint root by the observer

The observer currently operates in a dual-mode for finding executor checkpoints, it will check both under /var/run/thermos and under the task sandbox. The executor has only written checkpoints to the the task sandbox for quite awhile now.

At some point we should make the decision that the Observer reading from the global checkpoint root is no longer supported, at which point we should go back and clean up the observer to only look in task sandboxes for checkpoints.