Remove solver_nisland field from mjData.

PiperOrigin-RevId: 750527660
Change-Id: I6b34944c92483ea2abc774df75efd868255a2bac

src/engine/engine_forward.c

@@ -730,7 +730,6 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
       // solve using threads
       mj_solCG_island_multithreaded(m, d);
     }
-    d->solver_nisland = nisland;
   }
 
   // run solver over all constraints
@@ -751,9 +750,6 @@ void mj_fwdConstraint(const mjModel* m, mjData* d) {
     default:
       mjERROR("unknown solver type %d", m->opt.solver);
     }
-
-    // one (monolithic) island
-    d->solver_nisland = 1;
   }
 
   // save result for next step warmstart

src/engine/engine_io.c

@@ -1904,7 +1904,6 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
   memset(d->warning, 0, mjNWARNING*sizeof(mjWarningStat));
   memset(d->timer, 0, mjNTIMER*sizeof(mjTimerStat));
   memset(d->solver, 0, mjNSOLVER*mjNISLAND*sizeof(mjSolverStat));
-  d->solver_nisland = 0;
   mju_zeroInt(d->solver_niter, mjNISLAND);
   mju_zeroInt(d->solver_nnz, mjNISLAND);
   mju_zero(d->solver_fwdinv, 2);

src/engine/engine_print.c

@@ -1008,9 +1008,8 @@ void mj_printFormattedData(const mjModel* m, const mjData* d, const char* filena
   // SOLVER STAT
   if (d->nefc) {
     fprintf(fp, "SOLVER STAT\n");
-    fprintf(fp, "  solver_nisland = %d\n", d->solver_nisland);
     printVector("  solver_fwdinv = ", d->solver_fwdinv, 2, fp, float_format);
-    int nisland_stat = mjMIN(d->solver_nisland, mjNISLAND);
+    int nisland_stat = mjMAX(1, mjMIN(d->nisland, mjNISLAND));
     for (int island=0; island < nisland_stat; island++) {
       int niter_stat = mjMIN(mjNSOLVER, d->solver_niter[island]);
       if (niter_stat) {