remove "get_agents" rule in policy.json

Bug #1200933

keep the current API behaviour due to compatibility
and leave list op authz in new API version.

Change-Id: Ia0a9b8738fa8ffe913d2e2b1ef28232abb18340d

etc/policy.json

@@ -69,7 +69,6 @@
     "update_agent": "rule:admin_only",
     "delete_agent": "rule:admin_only",
     "get_agent": "rule:admin_only",
-    "get_agents": "rule:admin_only",
 
     "create_dhcp-network": "rule:admin_only",
     "delete_dhcp-network": "rule:admin_only",