'Remove' function not showing

I’m trying to remove an unwanted field in my Place > Settings > Custom Fields > ‘List of fields that will appear on add new Listing listing form’.

The unwanted field is post_content. The reason I am trying to remove it is that it doesn’t seem to offer me an option to ‘show only in these packages’. So I’ve used another text_area field in my form instead, one that includes a setting for ‘show only in these packages’.

My problem is I can’t seem to remove ‘post_content’ field from the ‘List of fields that will appear on add new Listing listing form’ because the red ‘Remove’ link is missing from it. Why would the ‘Remove’ link be missing?

Thanks!