Fix StackIt destination option relist

Removes `availability_zone` as a required relist option since it is not
actually required for relisting options. Now, whenever the `project`
option is being changed, all the options get reloaded based on it,
without it requiring the AZ option to be set.

config.ts

@@ -154,7 +154,7 @@ const conf: Config = {
     {
       name: "stackit",
       types: ["destination"],
-      requiredFields: ["availability_zone"],
+      requiredFields: [],
       relistFields: ["project"],
     },
   ],