We do things differently from most themes in order to give you all the options without dictating too much what we think you should need or not need.
In this case, a little CSS will do the trick to hide it and then the image will still be available for social sharing:
.single .attachment-post-thumbnail {
display: none;
}https://startupwp.com/topic/customizing-your-theme/
If that doesn’t work, please share the link to an example page.
Thank you.