Remove override get_schema

Since the patch[1] had been merged and release,
it's time to remove the override.
[1] https://review.openstack.org/389061
Change-Id: I8fc67250526d7d1c0c3442dc9172b8b97e8dac66

senlin/objects/fields.py

@@ -10,8 +10,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import copy
-
 from oslo_config import cfg
 from oslo_serialization import jsonutils
 from oslo_utils import strutils
@@ -382,20 +380,6 @@ class BaseEnum(fields.FieldType):
             return None
         return '\'%s\'' % value
 
-    def get_schema(self):
-        # TODO(Anyone): remove this override when the following patch is
-        # merged and released.
-        # https://review.openstack.org/389061
-        valid_values = copy.deepcopy(self._valid_values)
-        if self._nullable:
-            valid_values.append(None)
-
-        schema = {
-            'type': ['string'],
-            'enum': valid_values,
-        }
-        return schema
-
 
 class AdjustmentType(BaseEnum):