Old Install: Remove Post Type from post, without theme support.

I have a WP 4.8.1 install, and one of the posts has somehow been set as a Quote post (there’s an inverted comma logo proceeding its name in the post list). My theme doesn’t support post types, and this is the only post on the site to have a post type assigned.

I don’t have any options in the UI to remove the post type, presumably because the theme doesn’t support them.

What’s the solution to this? Enabling post types via functions.php, then setting that post to Unassigned, then removing post type support in functions.php?

I want to avoid inadvertently adding post type “unassigned” or similar to all my posts.

Thanks.