Fix: remove invalid `gitlab_branch_protection.unprotect_access_level` value `"no one"`.

The API response body contains the same error message as all the other invalid values:

$ curl -s -H "Authorization: Bearer $YOUR_TOKEN" \
  -H "Content-Type: application/json" \
  -X POST \
  https://gitlab.com/api/v4/projects/<project-id>/protected_branches/ \
  --data '{"name":"main","unprotect_access_level":0}' | jq .
{
  "error": "unprotect_access_level does not have a valid value"
}

Closes gitlab_branch_protection: unprotect_access_leve... (#1513 - closed)