Remove 'units' API altogether

Author: bradenmacdonald

src/openedx_content/api.py

@@ -17,4 +17,4 @@
 from .applets.publishing.api import *
 from .applets.sections.api import *
 from .applets.subsections.api import *
-from .applets.units.api import *
+from .applets.units.models import Unit  # Note this isn't a model. Should we move it?

src/openedx_content/applets/backup_restore/zipper.py

@@ -33,9 +33,6 @@
 from ..components import api as components_api
 from ..media import api as media_api
 from ..publishing import api as publishing_api
-from ..sections import api as sections_api
-from ..subsections import api as subsections_api
-from ..units import api as units_api
 from .serializers import (
     CollectionSerializer,
     ComponentSerializer,