Update unit tests for Organization.delete_manifest

Drop two test methods:

* `OrganizationTestCase.test_delete_manifest_200`
* `OrganizationTestCase.test_delete_manifest_202`

Add method `OrganizationTestCase.test_delete_manifest` to replace the two above.
This new test method is more thorough than the tests it replaces, as it covers
the following four possiblities:

* `synchronize` arg to `delete_manifest` is `True`, server returns HTTP 200.
* `synchronize` arg to `delete_manifest` is `True`, server returns HTTP 202.
* `synchronize` arg to `delete_manifest` is `False`, server returns HTTP 200.
* `synchronize` arg to `delete_manifest` is `False`, server returns HTTP 202.