Properly propagate job splitting for TaskChain with cascade KeepOutput=False

It should fix #5007 . 
I really cannot see the reason to have that code there, but as I may be wrong, I'm just commenting that out for the moment.

My test workflow is about to finish in my VM and the job splitting was correctly set. I want to run a few more tests tomorrow and if everything is fine, then we should try to push it to cmsweb-testbed this week.

@ticoann , please review :-)