We use "passive" for the differential performance analysis against concurrent GC. There is no reason to enable pacing when all GC cycles are STW.

Disabling pacing automatically gives us a few benefits:
 a) "Passive" becomes even more passive;
 b) Avoids unnecessary stalls for rendezvous with control thread, which might be the diagnosable source of stalls;
 c) Lists pacing options as ergonomically set with "passive", so users know how "passive" is different;
 d) Tests -ShenandoahPacing paths in the course of "passive" configuration testing.