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 '/'
I'm not sure whether anyone was using namespaces before.
I guess not because a workaround seems hard to come up with.

In my opinion, it should just be changed.
It will continue to work for everyone that did not use namespaces and enable to use namespaces for everyone else.