Remove transition logic for old tempest option renaming

This logic was required to adopt to the renaming in tempest[1] which
was done in 12.0.0 release. Now the minimum tempest version required
is much higher so we no longer need this compatibility consideration.

[1] ed6e586b9f8f0ada10af7711f297afa01b2b7754

Change-Id: I5d13f9146b00b2fcd5c2e96071ab914b8a56a089
diff --git a/neutron_tempest_plugin/api/test_routers.py b/neutron_tempest_plugin/api/test_routers.py
index 7758b1a..1430b81 100644
--- a/neutron_tempest_plugin/api/test_routers.py
+++ b/neutron_tempest_plugin/api/test_routers.py
@@ -38,9 +38,9 @@
     def resource_setup(cls):
         super(RoutersTest, cls).resource_setup()
         cls.tenant_cidr = (
-            config.safe_get_config_value('network', 'project_network_cidr')
+            CONF.network.project_network_cidr
             if cls._ip_version == 4 else
-            config.safe_get_config_value('network', 'project_network_v6_cidr'))
+            CONF.network.project_network_v6_cidr)
 
     @decorators.idempotent_id('c72c1c0c-2193-4aca-eeee-b1442640eeee')
     @tutils.requires_ext(extension="standard-attr-description",