Drop unused is_network_extension_supported

It has been unused since LBaaS tests were removed.

Change-Id: I1d4c92aead9014b12b86ea51851a7d047cce4d31
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>
diff --git a/heat_tempest_plugin/common/test.py b/heat_tempest_plugin/common/test.py
index c56ab40..567f2a0 100644
--- a/heat_tempest_plugin/common/test.py
+++ b/heat_tempest_plugin/common/test.py
@@ -18,7 +18,6 @@
 import fixtures
 from heatclient import exc as heat_exceptions
 from keystoneauth1 import exceptions as kc_exceptions
-from neutronclient.common import exceptions as network_exceptions
 from oslo_log import log as logging
 from oslo_utils import timeutils
 import testscenarios
@@ -294,13 +293,6 @@
             if net['name'] == net_name:
                 return net
 
-    def is_network_extension_supported(self, extension_alias):
-        try:
-            self.network_client.show_extension(extension_alias)
-        except network_exceptions.NeutronClientException:
-            return False
-        return True
-
     @staticmethod
     def _stack_output(stack, output_key, validate_errors=True):
         """Return a stack output value for a given key."""