Removed the group_tree_idname property from node types. This was a bit of a hack to associate node group types to specific tree types for node group operators. Since the node group operators now work independent from this property (see r56183) that property is no longer needed and just pollutes the bNodeType struct.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56185 954f8c5b-7b00-dc11-b283-0030488c597c