revert(emqx_conf): zone.external.max_topic_levels

revert part of changes in 289415b5aac36b1229b5fa3feccec90d2cf54e8e

etc/emqx.conf

@@ -772,8 +772,7 @@ zone.external.force_gc_policy = 16000|16MB
 ## Maximum topic levels allowed. 0 means no limit.
 ##
 ## Value: Number [0-65535]
-## Default: 7
-zone.external.max_topic_levels = 7
+## zone.external.max_topic_levels = 7
 
 ## Maximum QoS allowed.
 ##