@Mario is correct that there is a setting that will allow you to turn this display off. However, this setting doesn't seem to exist on the Google UI for remarketing tags, even though they do display this iframe (I think this is a bug on Google's end, as I imagine the "google_remarketing_only = true" flag was supposed to turn this iframe off and isn't working correctly).
I found out that you can also set this in the tracking JS by manually adding the flag "google_conversion_format = 3", like so:
<script type="text/javascript">
/* <![CDATA[ */
var google_conversion_id = 0123456789,
google_conversion_label = "XXXXXXXX",
google_custom_params = window.google_tag_params,
google_remarketing_only = true,
google_conversion_format = 3;
/* ]]> */
</script>
<script type="text/javascript" src="//www.googleadservices.com/pagead/conversion.js"></script>
This might be easier that regenerating the tags for some people, and solves the problem in the case that the UI doesn't support setting this option when generating the tags.