Remove charge group code from forcerec

Remove charge group code from forcerec

Removed sevearal charge group members from t_forcerec.
Removed the cginfo intra-cg exclusion flags, as it is always true
for single atoms.
Removed loops over charge groups from init_cginfo_mb().

Change-Id: Ic91ea78dc341dc9972567a488294cb3d262ef9f6