MuMonitor: Remove unnecessary get of HltDecReports

Marco Cattaneorequested to merge
fixMuMonitorForPassthru into master

MuMonitor was retrieving HltDecReports from the TES, even though it was not using them. This MR removes it, allowing MuMonitor to work also on data taken in HLT passthrough mode where the HltDecReports are absent. See discussion in Brunel!484 (merged)

@rmatev FYI

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