removed currently non-existing STLExporter

git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@81 bbaffbd6-741e-11dd-a85d-61de82d9cad9

pycam/Exporters/__init__.py

-list = [ "SimpleGCodeExporter", "SVGExporter", "STLExporter" ]
+list = [ "SimpleGCodeExporter", "SVGExporter"]
 __all__ = list