Remove transformation between base and base_link

The following line defines a transformation between the base according to ROS conventions and according to the UR control box, which differ 180 degrees along the Z axis.

https://github.com/airo-ugent/airo-drake/blob/14110cb99553722324b3ae1b572659471067635d/airo_drake/building/manipulator.py#L9

This should no longer be used, the convention should be that the base frame from the URDF corresponds to the base frame from the robot manufacturer (in this case the UR control box).