chore: remove duplicated haproxy config section

the mgmt and dashbaord endpoints are both merged 18083 now
no need to declare twice

.ci/docker-compose-file/haproxy/haproxy.cfg

@@ -30,24 +30,12 @@ defaults
 ##----------------------------------------------------------------
 ## API
 ##----------------------------------------------------------------
-frontend emqx_mgmt
-   mode tcp
-   option tcplog
-   bind *:18083
-   default_backend emqx_mgmt_back
-
 frontend emqx_dashboard
    mode tcp
    option tcplog
    bind *:18083
    default_backend emqx_dashboard_back
 
-backend emqx_mgmt_back
-    mode http
-    # balance static-rr
-    server emqx-1 node1.emqx.io:18083
-    server emqx-2 node2.emqx.io:18083
-
 backend emqx_dashboard_back
     mode http
     # balance static-rr