Fix ray-related warning printed by sdformat while parsing

[traversaro]

While parsing the SDF generated out of the URDF files of ergoCub in Gazebo via sdformat, a warning is printed:

Warning [parser.cc:833] XML Attribute[name] in element[ray] not defined in SDF, ignoring.

See ami-iit/rod#26 .

It turns out that we are adding a name attribute to the ray element of the sdf, but this attribute is not supported, nor parsed (see http://sdformat.org/spec?ver=1.10&elem=sensor#sensor_ray). The sensor element (parent of ray) has already a name attribute, and that can be used to identify the sensor. I guess this happened as a copy&paste error due to thefact the camera element instead supports an optional name element (see http://sdformat.org/spec?ver=1.10&elem=sensor#sensor_camera), even if to be honest I find it a bit confusing, and probably we can omit it there as well.

To fix the warning, we can just drop the name attribute, that anyhow was not parsed.