move dispose code to disposeNumberPicker, https://github.com/phetsims/sun/issues/121
move dispose code to disposeNumberPicker, https://github.com/phetsims/sun/issues/121

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/sun,phetsims/sun,phetsims/sun