Remove recalculateCenter() calls from EarthManipulator::setViewpoint()

As noted in this discussion:

http://forum.osgearth.org/recalculateCenter-in-EarthManipulator-setViewpoint-td7580074.html

the calls to EarthManipulator::recalculateCenter() are causing problems for several people, myself included.  Everyone who has had this problem ends up commenting lines out, so here's an official pull request to officially remove them.

Thanks,

--Chris W.