Remove legacy function declaration in user_objects.h
Remove legacy function declaration in user_objects.h

PiperOrigin-RevId: 484464868
Change-Id: I386960c9f2057f4b1a1b6d818ed950ea8439202d

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco