Removed detail rule from showAPIError

refs: https://github.com/TryGhost/Team/issues/1121

- showAPIError is a method intended for formatting errors from the Ghost API
- Ghost API Errors do not have a detail field, therefore this code was redundant
- there are also no related tests
- removing now because I'm trying to cleanup and streamline all our error handling code

ghost/admin/app/services/notifications.js

@@ -180,8 +180,6 @@ export default class NotificationsService extends Service {
             msg = GENERIC_ERROR_MESSAGE;
         } else if (resp instanceof String) {
             msg = resp;
-        } else if (!isBlank(resp?.detail)) {
-            msg = resp.detail;
         } else if (!isBlank(resp?.message)) {
             msg = resp.message;
         }