refactor: remove not_required

[domoritz]

For #730

Doesn't quite work.

draco/asp/generate.lp:6:1-85: error: unsafe variables in:
  0<=#count{0,attribute((N,A),E,V):attribute((N,A),E,V):domain((N,A),V)}<=1:-[#inc_base];#p_entity(#b(N),#p,#b(E));not required((N,A)).
draco/asp/generate.lp:6:18-19: note: 'A' is unsafe