Set default body position to (0, 0, 0).

The "undefined" body position is a remnant of the now-removed `compiler/coordinate` attribute, see [documentation in version 2.3.3](https://mujoco.readthedocs.io/en/2.3.3/modeling.html#cframe) (or older).

PiperOrigin-RevId: 676361485
Change-Id: I104c8da2a988a0430eb85fff8b9b60f8c1e08582

src/user/user_init.c

@@ -78,7 +78,6 @@ void mjs_defaultBody(mjsBody* body) {
   memset(body, 0, sizeof(mjsBody));
 
   // body frame
-  body->pos[0] = mjNAN;
   body->quat[0] = 1;
 
   // inertial frame