world.h: remove initXml function, tinyxml include

[scpeters]

The urdf::World::initXml function is never implemented
and causes an unnecessary tinyxml dependency.
So remove that function and the tinyxml include.

[jslee02]

Can we merge this? This PR is blocking ros/urdfdom#99 (build error).

[traversaro]

Just to understand: we are keeping the void initXml(TiXmlElement* config); instead of removing it to preserve the ABI, right?

[jslee02]

Just to understand: we are keeping the void initXml(TiXmlElement* config); instead of removing it to preserve the ABI, right?

I believe so.

Also, for the consistency with urdfdom, I would like to suggest either switching to TinyXML2 for urdfdom_headers or removing initXml because ros/urdfdom#99 will break the API anyway. I'm inclined to removing initXml since it's not implemented.

[clalancette]

I did a little bit of code archeology here, and as far as I can tell, initXml has never been implemented for the World class. Thus, there is no way any downstream users can be dependent on it; they would always fail to compile. Additionally, back in 2012, in commit ea2982a, all of the other initXml methods on all of the other classes were removed, presumably for the same reasons. Finally, if you look at the World class closely, it is clearly meant to be used as a header-only class. Thus, I would strongly suggest that we just kill off the initXml method here, and remove the tinyxml dependency from this header file. I've done that in 468ad1d ; @scpeters feel free to cherry-pick that here.