[CSL-1592] Get rid of nonsensical 'fork' in us worker
[CSL-1592] Get rid of nonsensical 'fork' in us worker

That fork doesn't make any sense as was discussed multiple times in
various channels.
Action inside 'localOnNewSlotWorker' is forked anyway.
And 'checkForUpdate' is the last action in this worker.

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype