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

unity/Runtime/Importer/MjcfImporter.cs

@@ -296,9 +296,6 @@ public class MjcfImporter {
       case "tendon":
         equalityType = typeof(MjTendonConstraint);
         break;
-      case "distance":
-        equalityType = typeof(MjDistance);
-        break;
       default:
         Debug.Log($"The importer does not yet support equality <{node.Name}>.");
         break;