Remove mj_jacDifPairCount by incorporating the counting into mj_jacDifPair

PiperOrigin-RevId: 884366070
Change-Id: I7490ca205a315bc661414e32c91ad1ec753a8fe7

src/engine/engine_core_util.c

@@ -456,6 +456,11 @@ int mj_jacDifPair(const mjModel* m, const mjData* d, int* chain,
     return 0;
   }
 
+  // count-only mode
+  if (!jacdifp && !jacdifr && !jac1p && !jac1r) {
+    return NV;
+  }
+
   // sparse case
   if (issparse) {
     // simple: fast processing