remove xiaoxu from pr reviewer list (#14571)

Author: xiaoxuqi-ms

.github/pull_request_assignment.yml

@@ -171,13 +171,6 @@
     reviewers:
       - zhenglaizhang
 
-- rule:
-    paths: 
-      - "specification/azurestack/**"
-      - "specification/automation/**"
-    reviewers:
-      - xiaoxuqi-ms
-
 - rule:
     paths:
       - "documentation/**"