remove leading '/' from DEFAULT_PLANNING_SCENE_SERVICE

 otherwise planning_scene_rviz_plugin fails with 'Failed to call service /get_planning_scene ...' if move_group is in a namespace different from '/'
Because that would result in ~get_planning_scene, i.e. a _private_ name, but we need a _relative_ name.
(It worked before because global names are global in both cases)