Fixes #38102 - Auth token of flatpak-remote shouldn't be returned by API

[sjha4]

What are the changes introduced in this pull request?

Don't return token of flatpak remote on the API

Considerations taken when implementing this change?

What are the testing steps for this pull request?

Go to katello/api/v2/flatpak_remotes/ and make sure you don't see the authentication token.

[vsedmik]

Tested the patch locally and looks good to me - no token was returned:

$ curl -sku lojza:changeme -H "Content-type: application/json" -X GET https://satellite.redhat.com/katello/api/flatpak_remotes/2 | jq
{
  "name": "CUzimMsarr",
  "url": "https://flatpaks.redhat.io/rhel/",
  "description": null,
  "username": "1234567|wonderful-tokenauth",
  "seeded": false,
  "registry_url": "https://registry.redhat.io/",
  "id": 2,
  "organization_id": 4,
  "organization": {
    "name": "LBrOQnkgYtXs",
    "label": "LBrOQnkgYtXs",
    "id": 4
  }
}