Improve CRB code:

- remove mj_crbSkip since this was only utilized during compile time to compute m->dof_m0, replaced with mj_crbDiag to compute only the diagonals for dof_m0,
- change row backward pass to forward pass to optimize processing diagonal blocks,
- and various code refactoring.

PiperOrigin-RevId: 556799924
Change-Id: I0a18d7a45862860fdddd7150540ddc8d64b01b29

src/engine/engine_core_smooth.h

@@ -42,9 +42,6 @@ MJAPI void mj_transmission(const mjModel* m, mjData* d);
 
 //-------------------------- inertia ---------------------------------------------------------------
 
-// composite rigid body inertia algorithm, with skip
-void mj_crbSkip(const mjModel* m, mjData* d, int skipsimple);
-
 // composite rigid body inertia algorithm
 MJAPI void mj_crb(const mjModel* m, mjData* d);