Remove outdated debug macro from GlobalChi2Fitter

Alexander J Pflegerrequested to merge
apfleger/athena:gx2f-debugmode into main

The variable m_iterations is nowhere accessed or modified.

$ grep -IRn "GXFDEBUGCODE"
Tracking/TrkFitter/TrkGlobalChi2Fitter/TrkGlobalChi2Fitter/GlobalChi2Fitter.h:7://#define GXFDEBUGCODE
Tracking/TrkFitter/TrkGlobalChi2Fitter/TrkGlobalChi2Fitter/GlobalChi2Fitter.h:230:      #ifdef GXFDEBUGCODE

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