Remove "usedFallback" boolean from (pre|post)OpenFile signals

[makortel]

After cms-sw/cmssw#35505 the "useFallback" information is not available yet for preOpenFile signal, and delivering it via postOpenFile signal would be a bit cumbersome. Currently this boolean is used only by Tracer in (pre|post)OpenFile.

After cms-sw/cmssw#35505 Tracer is also the only user of the boolean in (pre|post)CloseFile signals ( StatisticsSenderService gets it by other means), so we could consider removing the boolean also from those signals (if not deemed useful in Tracer).

Done in cms-sw/cmssw#36400

[makortel]

We agreed that Tracer doesn't really need this information, so we could just remove this boolean also for (pre|post)CloseFile signals.