Minor PrimitivePipeline cleanup.

This change is basically a no-op but makes the code less confusing.  Eagle eye @shunter noticed an unused variable in PrimitivePipeline a while ago that he thought might be a bug.  Turns out it wasn't a bug but just sloppy coding on my part.

`PrimitivePipeline.unpackCombineGeometryParameters` was not including the cloned `modelMatrix` parameter as part of the result.  Because of that `PrimitivePipeline.combineGeometry` was unnecessarily creating a dummy object in order to clone it.  This is no longer needed.

There was also an uneeded clone of the resulting `modelMatrix` in Primtive.js, the result matrix is the same instance that is passed into `PrimitivePipeline.combineGeometry` so no clone is needed.