Fix usage of locked WriteDecorHandleKey in BoostedJetTaggers (ATLJETMET-1715)
Follow up to !71273 (merged), as there were still some WriteDecorHandleKey
usage leading to
ERROR SG::ExcStoreLocked: Attempted to modify auxiliary data in a locked store: `::Tau21_wta' (1203)
Fixed by introducing void decorateJSSRatios( const xAOD::JetContainer& jets ) const;
+ moving ParentJetNTrkPt500
from decoration to direct return argument
Also cleans up unnecessary m_jetContainer_key