Removed ._filter from Reference. Updated related() to remove ._sort from new Reference.
Removed ._filter from Reference. Updated related() to remove ._sort from new Reference.

* ._filter is not being used. If we keep it in Reference, it has to be updated with related() call. Better remove it until we actually need it.
* Remove sort after with new Reference from the related() call.

informatics-isi-edu/ermrestjs,informatics-isi-edu/ermrestjs,informatics-isi-edu/ermrestjs