set on_delete to DO_NOTHING in RelatedSetRel

Currently, attempting to delete() an object in a RelatedSetRel results in an AttributeError on RelatedSetRel. This simply defines it to DO_NOTHING. Could be made customisable but I don't know if we'd want it to perform any of the other deletion actions.

This doesn't take care of actually removing the object from the RelatedSetField - it will remain in the list of ids although won't be returned in the queryset.