Remove api_rate_limit config option

The option was disabled by default in Havana with commit
cfdc59860ef1482e3b064f90f719ec9e484590ce and it's usage
was removed in Newton with commit
5a64f5782452d59d6ac25379a2420585e861af5a, so remove the
option since it's not longer used.

A release note is added to note the removal of the option
which was broken and confusing anyway.

Part of blueprint remove-legacy-v2-api-code

Change-Id: I23a390e5fa269c6a4a741bd414153764ea37e79c

nova/conf/api.py

@@ -17,28 +17,6 @@ from oslo_config import cfg
 
 
 auth_opts = [
-    cfg.BoolOpt("api_rate_limit",
-            default=False,
-            deprecated_for_removal=True,
-            deprecated_group="DEFAULT",
-            help="""
-*DEPRECATED*
-
-Determines whether rate limiting for the API is per-user. This option is only
-used by v2 API. Rate limiting is removed from v2.1 API.
-
-* Possible values:
-
-    True, False (default)
-
-* Services that use this:
-
-    ``nova-api``
-
-* Related options:
-
-    None
-"""),
     cfg.StrOpt("auth_strategy",
             default="keystone",
             choices=("keystone", "noauth2"),

releasenotes/notes/remove-api-rate-limit-option-91a17e057081381a.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - The ``api_rate_limit`` configuration option has been removed. The option
+    was disabled by default back in the Havana release since it's effectively
+    broken for more than one API worker. It has been removed because the legacy
+    v2 API code that was using it has also been removed.