Remove dead code from display.js

chunkClass.unique is never set (chunkClass.prototype.unique might be
set), so this code was dead.
Reuse SceneJS_Chunk.init when constructing new chunks

This removes some duplicate code. "type" is set on the chunk prototype,
so there's no need to set it again in the constructor.
Merge pull request #318 from NVIDIA/cleanup

Cleanup