InDetGlobalPrimaryVertexMonAlg: remove unused ReadHandleKey leading to per-event warnings

Remove the unused read-handle key m_vxContainerNameWithoutBeamConstraint. Removes the per-event warning

SGInputLoader WARNING unable to find proxy for  ( 'xAOD::VertexContainer' , 'StoreGateSvc+VxPrimaryCandidateWithBeamConstr

that we see when running MT.