Remove distance constraints.

- `distance` constraints were not very useful and increased the contact buffer within the constraint solver, violating a core design principle. Therefore they have been removed.
- The enum value `mjEQ_DISTANCE` is retained, but errors will be thrown in the parser and engine if these constraints are selected.

PiperOrigin-RevId: 467050101
Change-Id: Ie72a07e4fe338eea5107004a9fc0f5e47eb0c3b7

src/engine/engine_io.c

@@ -1370,13 +1370,7 @@ const char* mj_validateReferences(const mjModel* m) {
         }
         break;
       case mjEQ_DISTANCE:
-        if (obj1id >= m->ngeom || obj1id < 0) {
-          return "Invalid model: eq_obj1id out of bounds.";
-        }
-        if (obj2id >= m->ngeom || obj2id < 0) {
-          return "Invalid model: eq_obj2id out of bounds.";
-        }
-        break;
+        return "distance equality constraints are no longer supported";
       case mjEQ_WELD:
       case mjEQ_CONNECT:
         if (obj1id >= m->nbody || obj1id < 0) {