I couldn’t figure out where that thumbnail is coming from, I tried changing any theme option that might have an effect and couldn’t duplicate the problem.
What you might try is temporarily deactivating all of your plugins and see if that makes the thumbnail go away. If it does, you can then try reactivating them one-by-one to see which one is causing the problem.
If that doesn’t help, you can add a CSS rule to hide that thumbnail. I wouldn’t be making changes to the theme’s style.css file. If the theme gets updated because of feature enhancements, bug fixes, or security patches, or if the theme has to be updated because of a change to the WordPress core, then your changes will be lost. Instead, either create a child theme or use a Custom CSS plugin like Jetpack or Custom CSS Manager. If you’re just making CSS changes, then using a CSS plugin is easier.
Try adding this CSS rule:
.attachment-post-thumbnail {
display: none;
}
