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

See merge request !6234