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_core_smooth.c

@@ -1904,11 +1904,13 @@ void mj_rnePostConstraint(const mjModel* m, mjData* d) {
 
     case mjEQ_JOINT:
     case mjEQ_TENDON:
-    case mjEQ_DISTANCE:
       // increment 1 row
       i++;
       break;
 
+    case mjEQ_DISTANCE:
+      mju_error("distance equality constraints are no longer supported");
+
     default:
       mju_error_i("Unknown constraint type type %d", m->eq_type[id]);    // SHOULD NOT OCCUR
     }