Remove molsetup ring attributes that are not needed

[diogomart]

After the refactoring of flexibility.py (32f2469) that currently lives in the residue_molsetups branch, all attributes of rings stored in the molsetup can be removed: corner_flip, graph, and is_aromatic. The only information that is needed are the indices of the atoms that constitute the ring, which have been used as key to the molsetup.rings dictionary.