caddy-frontend: Drop unused parameter nginx-domain

It's from old approach, which is not going to be implemented.

software/caddy-frontend/instance-caddy-input-schema.json

@@ -50,12 +50,6 @@
       "title": "Duration of the graceful shutdown period. Warning: Changing the parameter will result in restarting Caddy process.",
       "type": "integer"
     },
-    "nginx-domain": {
-      "description": "Base Domain for create subdomains (ie.: example2.com) for websocket, notebook and eventsource.",
-      "pattern": "^([a-zA-Z0-9]([a-zA-Z0-9\\-]{0,61}[a-zA-Z0-9])?\\.)+[a-zA-Z]{2,6}$",
-      "title": "[NOT IMPLEMENTED] Nginx Domain",
-      "type": "string"
-    },
     "public-ipv4": {
       "description": "Public ipv4 of the frontend (the one Caddy will be indirectly listening to).",
       "title": "Public IPv4",