Use the thread safe slimCopy in EMBremCollectionBuilder

Since we do not set up m_setPersistificationHints to true anyhow

https://gitlab.cern.ch/atlas/athena/blob/2485766e28e9f1a9cb5c3661b4c873c4a9550ec5/Tracking/TrkTools/TrkTrackSlimmingTool/TrkTrackSlimmingTool/TrackSlimmingTool.h

We can use avoid the thread unsafe interface .