Drop remaining deprecated upgrade_levels option for nova-cert

The nova-cert service was removed during Pike cycle by 2bcee77e3 and
the upgrade_levels option for this service was formally deprecated
during rocky by f0d2925bc71 . The other upgrade_levels options which
were deprecated at the same time were already removed.

Change-Id: I385dc41a3a69c51d60acced21cfdf6c6dd0cc724

nova/conf/upgrade_levels.py

@@ -55,24 +55,6 @@ Possible values:
 * By default send the latest version the client knows about
 * 'auto': Automatically determines what version to use based on
   the service versions in the deployment.
-* A string representing a version number in the format 'N.N';
-  for example, possible values might be '1.12' or '2.0'.
-* An OpenStack release name, in lower case, such as 'mitaka' or
-  'liberty'.
-"""),
-    cfg.StrOpt("cert",
-        deprecated_for_removal=True,
-        deprecated_since='18.0.0',
-        deprecated_reason="""
-The nova-cert service was removed in 16.0.0 (Pike) so this option
-is no longer used.
-""",
-        help="""
-Cert RPC API version cap.
-
-Possible values:
-
-* By default send the latest version the client knows about
 * A string representing a version number in the format 'N.N';
   for example, possible values might be '1.12' or '2.0'.
 * An OpenStack release name, in lower case, such as 'mitaka' or

releasenotes/notes/remove-upgrade_levels-cert-5917509f849b1648.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - |
+    The deprecated ``[upgrade_levels] cert`` option has been removed.