Code clean-up of CLI test "cinder list-extensions"

Redundant code is removed:
list-extensions

Change-Id: Ia0ba0c897c36ceea8d909276d019f1736a86d879
diff --git a/tempest/cli/simple_read_only/test_cinder.py b/tempest/cli/simple_read_only/test_cinder.py
index 9001302..a5a81c0 100644
--- a/tempest/cli/simple_read_only/test_cinder.py
+++ b/tempest/cli/simple_read_only/test_cinder.py
@@ -96,7 +96,6 @@
         self.cinder('type-list')
 
     def test_cinder_list_extensions(self):
-        self.cinder('list-extensions')
         roles = self.parser.listing(self.cinder('list-extensions'))
         self.assertTableStruct(roles, ['Name', 'Summary', 'Alias', 'Updated'])