Migrate away from Ligand Expo

Ligand Expo is being retired: https://www.rcsb.org/news/feature/6824d7079d8e3e741c686889

Update our make-mmcif code that currently uses Ligand Expo to instead use URLs like https://files.rcsb.org/pub/pdb/refdata/chem_comp/S/HIS/HIS.cif