PIG-3561: Clean up PigStats and JobStats after PIG-3419 (cheolsoo)


git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1540855 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd