Fix non-running deletion tests

Many of the tests in deleting.rb had accidentally been disabled. This
change re-enables the tests and removes the flakey "around" block which
had been causing them not to run.

The disabled tests were caused by a false-positive test for
relation[:dataset] which intended to check whether the association was a
virtual association rather than a real relationship. In practice, the
passed relationships are never virtual relationships, so the test was
unnecesary. This change removes the check entirely. Additionally, to
prevent this regressing again, if any associated
object IDs are provided, the tests will fail with an error if there are
no associated objects to test recursive deletion against.

Apps_controller and Service_Plans_Controller both implement their own
deletion logic which differs (intentionally) from the shared deletion
behaviour, so both of the related specs of these classes no longer include
the shared examples from deleting.rb.