xAODRootAccess: Remove calls to IAuxStoreIO::selectAux.

Trying to remove the (thread-problematic) AuxSelection member from AuxContainerBase.

Here, we remove calls to IAuxStoreIO::selectAux. Instead, we do the selection directly where it's needed, from what's in m_auxItemList.

For now, keep the selectAux functionality in TAuxStore.