Fix object pose by removing redundant z_min correction.

The min_z is subtracted twice.

First the mesh vertices are offset so that z=0 is the minimum for the mesh (in https://github.com/shadow-robot/tabletop/blob/a107a368d019557b69b77f281561fd80a8ac528b/src/object/ObjectRecognizer.cpp#L184)

Then the pose found for the object is offset again, leading to a wrong position for the object, as shown in the following image:

![rviz_object_recognition_bad_z](https://cloud.githubusercontent.com/assets/3338520/2606314/7758c3d2-bb4e-11e3-9207-1a865adade41.png)

Using the correction in this pull request the pose is in the right place (see the cam RGB image on the right side of the rviz screen):

![rviz_object_recognition_good_z](https://cloud.githubusercontent.com/assets/3338520/2606338/bb206da4-bb4e-11e3-9d18-cb43faa3afd0.png)