Remove this useless delete method for Twinnable_ManyMany :
 - if $cascade === false, parent::delete() only call $this->delete_related()
 - if these's twin models, $this->delete_related() doesn't delete relationship entries
Merge pull request #120 from novius-os/bugfix/manymany-twinnable-delete-relationship-entries

Remove this useless delete method for Twinnable_ManyMany :