Make PVS ignore duplicate view subscriber

[ElectroJr]

Changes PvsSystem.GetSessionViewers() so that it just ignores a view subscription if the player is already attached to the entity. Also changes SharedEyeSystem.SetTarget() so that it removes the old target from the view subscription. I'm not sure if it was intentional. but adding them automatically, without also removing them automatically could've lead to players running around with a bunch of old view subs.