Delete remaining code related to now-removed distance equality constraint.

PiperOrigin-RevId: 513391064
Change-Id: I47fc344708a580c1e99bd2753113aaa408812966

src/engine/engine_core_smooth.c

@@ -908,7 +908,7 @@ void mj_transmission(const mjModel* m, mjData* d) {
             }
           }
 
-          // excluded contact, get sparse or dense Jacobian, accumulate
+          // excluded contact in gap: get sparse or dense Jacobian, accumulate
           else if (con->exclude == 1) {
             counter++;
 
@@ -2034,10 +2034,6 @@ void mj_rnePostConstraint(const mjModel* m, mjData* d) {
       i++;
       break;
 
-    case mjEQ_DISTANCE:
-      mju_error("distance equality constraints are no longer supported");
-      break;
-
     default:
       mju_error_i("Unknown constraint type type %d", m->eq_type[id]);    // SHOULD NOT OCCUR
     }