Do not try to update a repository's content_type

From IRC:

    <ichimonji10> Hey all. Is it possible to update the content_type of a
    repository?
    <jsherrill> ichimonji10: its not
    <jsherrill> you have to delete and re-create

This fixes a failing Jenkins test. Updated test output:

    $ nosetests tests/foreman/api/test_repository.py -m test_update
    ....S........
    ----------------------------------------------------------------------
    Ran 13 tests in 29.033s

    OK (SKIP=1)
Merge pull request #1797 from Ichimonji10/repo-update

Do not try to update a repository's content_type