fix cloning of shape id property by split (#186)

cvat/apps/engine/static/engine/js/shapeSplitter.js

@@ -58,6 +58,8 @@ class ShapeSplitter {
                 attributes: curAttributes,
             }));
 
+            // don't clone id of splitted object
+            delete exported.id;
             let prevExported = Object.assign({}, exported);
             let curExported = Object.assign({}, exported);
             prevExported.shapes = prevPositionList;
@@ -71,4 +73,4 @@ class ShapeSplitter {
             return [exported];
         }
     }
-}
+}