use net mtu api def from neutron-lib

The net mtu extension's API definition was moved into neutron-lib with
commit I41d5b399a6bd02f89f2e60c5ccb3fc0944b61255
This patch consumes it by removing the rehomed code and using the
APIExtensionDescriptor for the parent class of the extension.

NeutronLibImpact

Change-Id: I0bc7ba5e1e267183e9b440bedcdbe373e635297e

neutron/extensions/netmtu.py

@@ -12,39 +12,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from neutron_lib.api.definitions import network_mtu as apidef
 from neutron_lib.api import extensions
 
 
-MTU = 'mtu'
-EXTENDED_ATTRIBUTES_2_0 = {
-    'networks': {
-        MTU: {'allow_post': False, 'allow_put': False,
-              'is_visible': True},
-    },
-}
-
-
-class Netmtu(extensions.ExtensionDescriptor):
+class Netmtu(extensions.APIExtensionDescriptor):
     """Extension class supporting network MTU."""
 
-    @classmethod
-    def get_name(cls):
-        return "Network MTU"
-
-    @classmethod
-    def get_alias(cls):
-        return "net-mtu"
-
-    @classmethod
-    def get_description(cls):
-        return "Provides MTU attribute for a network resource."
-
-    @classmethod
-    def get_updated(cls):
-        return "2015-03-25T10:00:00-00:00"
-
-    def get_extended_resources(self, version):
-        if version == "2.0":
-            return EXTENDED_ATTRIBUTES_2_0
-        else:
-            return {}
+    api_definition = apidef