Wait for server termination in server affinity tests

These tests were not waiting for server termination and
leading to failures in another serial tests which expects
no server running on the compute nodes.
Using method from parent class which already waits for
server deletion.

Closes-Bug: #2097594
Change-Id: Iea6ea61c5b827be705777cca5b972a5441a69540
diff --git a/tempest/serial_tests/api/compute/admin/test_server_affinity.py b/tempest/serial_tests/api/compute/admin/test_server_affinity.py
index 4400130..71738bc 100644
--- a/tempest/serial_tests/api/compute/admin/test_server_affinity.py
+++ b/tempest/serial_tests/api/compute/admin/test_server_affinity.py
@@ -71,7 +71,7 @@
         body, servers = compute.create_test_server(
             self.os_primary, networks='none', **kwargs)
         for server in servers:
-            self.addCleanup(self.servers_client.delete_server, server['id'])
+            self.addCleanup(self.delete_server, server['id'])
         return body
 
     def _create_server_group(self, **kwargs):