Fix behaviour of the event cutflow in case a single event selection is provided
Bug reported by @arej: the event cutflow histogram is flat when a single event selection is defined.
This is because the algorithm responsible for the cutflow is scheduled after the hard event filter, and therefore only sees events passing the full selection. In the case where there are multiple event selections defined, the event filter is turned off to allow all of them to be executed, and only at the very end (after all cutflow algs) is the event filter applied based on the logical-OR of all individual selections. Therefore that case is not affected by the "bug".
Solution: also make the single-selection case go through that logic, disabling the event filter, producing the histogram, and only then filtering.