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/xml/xml_native_reader.cc

@@ -1328,9 +1328,7 @@ void mjXReader::OneEquality(XMLElement* elem, mjCEquality* pequality) {
       break;
 
     case mjEQ_DISTANCE:
-      ReadAttrTxt(elem, "geom1", pequality->name1, true);
-      ReadAttrTxt(elem, "geom2", pequality->name2, true);
-      ReadAttr(elem, "distance", 1, pequality->data, text);
+      throw mjXError(elem, "support for distance equality contraints was removed in MuJoCo 2.2.2");
       break;
 
     default:                    // SHOULD NOT OCCUR