Clean up unused members in SolenoidTest.h (!74333) · Merge requests · atlas / athena · GitLab

Follow up to !74269 (merged), fixes compilation warning in clang17 release https://atlas-sw-build-logs.web.cern.ch/nightlywww/CLANGWebArea/ardoc_web_areamain64BE9C17AthenaOpt/ARDOC_Log_2024-09-16T2101/MagneticField_MagFieldUtils.html