Restore legacy center-of-mass computation when `exactmeshinertia` is false.
Restore legacy center-of-mass computation when `exactmeshinertia` is false.

PiperOrigin-RevId: 464511268
Change-Id: Ic7c640d01e58dba5049e9fcb2c1b75d6bff4f956

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