Restore gl_dropdown.js changes
Restores fieldName
to allow only string values. fieldName
was changed to accept function as a value in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581 Since the approach changed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/687 there's no need to accept function as value.
Backport changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/687
EE Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/919