quick fix for clearJointData() in JS scripts

davedub is working on an animation tool and noticed that clearJointData() was broken:

https://alphas.highfidelity.io/t/first-steps-in-character-animation-updates/752/39

The problem was that Model::setJointState() was checking the priority incorrectly (when priority is 0.0 then special things happen).  Since the JointState class already handles this priority check correctly I removed the logic  from Model.