Remove admin_only kwarg from rbac_rule_validation decorator

The ``admin_only`` kwarg has been removed from ``rbac_rule_validation``
decorator because it is no longer used by any tests. Besides that,
it should not be used because Patrole is dedicated to RBAC testing and
an admin-only check is not RBAC because it does not use ``oslo.policy``
library.

Change-Id: Iee4881a8756cf2f2a0a4520dd450776a4f1e873c
diff --git a/releasenotes/notes/remove-admin-only-kwarg-919f1a4797318a33.yaml b/releasenotes/notes/remove-admin-only-kwarg-919f1a4797318a33.yaml
new file mode 100644
index 0000000..21b8eb0
--- /dev/null
+++ b/releasenotes/notes/remove-admin-only-kwarg-919f1a4797318a33.yaml
@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    The ``admin_only`` kwarg has been removed from ``rbac_rule_validation``
+    decorator because it is no longer used by any tests. Besides that,
+    it should not be used because Patrole is dedicated to RBAC testing and
+    an admin-only check is not RBAC because it does not use ``oslo.policy``
+    library.