Remove support for generation of virtual sites to replace aromatic rings in standard residues - Redmine #3254

Deprecated in GROMACS 2019

These are thought to produce artefacts under some circumstances (unpublished results), were never well tested, are not widely used, and we need to simplify pdb2gmx.

(from redmine: issue id 3254, created on 2019-12-19 by eirrgang)