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/base.py b/neutron_tempest_plugin/api/base.py
index e17cf5e..4bcc6d2 100644
--- a/neutron_tempest_plugin/api/base.py
+++ b/neutron_tempest_plugin/api/base.py
@@ -595,15 +595,11 @@
             # Generate subnet CIDRs starting from configured values
             ip_version = ip_version or cls._ip_version
             if ip_version == const.IP_VERSION_4:
-                mask_bits = mask_bits or config.safe_get_config_value(
-                    'network', 'project_network_mask_bits')
-                cidr = netaddr.IPNetwork(config.safe_get_config_value(
-                    'network', 'project_network_cidr'))
+                mask_bits = mask_bits or CONF.network.project_network_mask_bits
+                cidr = netaddr.IPNetwork(CONF.network.project_network_cidr)
             elif ip_version == const.IP_VERSION_6:
-                mask_bits = config.safe_get_config_value(
-                    'network', 'project_network_v6_mask_bits')
-                cidr = netaddr.IPNetwork(config.safe_get_config_value(
-                    'network', 'project_network_v6_cidr'))
+                mask_bits = CONF.network.project_network_v6_mask_bits
+                cidr = netaddr.IPNetwork(CONF.network.project_network_v6_cidr)
             else:
                 raise ValueError('Invalid IP version: {!r}'.format(ip_version))