There is no secret that Elegant themes DIVI and their subtheme EXTRA are the best and most powerful WordPress flexible themes available.
At the same time, you still need some little tweaks and adjustments to make it perfect for all situations. 

Elegant Themes developers didn't provide the option to disable featured images from appearing at the top of content type "Project" in their Extra theme. It's not flexible. There is the quick solution how to easy get it done.

Extra theme Hide Featured Image on Post

However, it's still available for individual posts via Extra settings per post "Hide Featured Image on Post".

Extra Theme Options

If you would like to hide the featured images on all posts or all projects in Extra theme then you can add the following lines in the custom CSS section "Custom CSS" from the WordPress Admin menu Extra-Theme Options-General.

Remove/hiding the Featured Images

If you would like to hide the featured images on all posts in Extra theme then add one followind line in custom CSS section "Custom CSS" from WordPress Admin menu Extra-Theme Options-General.

.single .post-thumbnail.header { display: none; }

If you would like to hide the featured images on all projects in Extra theme then add one followind line in custom CSS section "Custom CSS" from WordPress Admin menu Extra-Theme Options-General.

.single-project .post-thumbnail { display: none; }

If you would like to hide the featured images on all posts and on all projects in Extra theme then add two followind lines in custom CSS section "Custom CSS" from WordPress Admin menu Extra-Theme Options-General.

.single .post-thumbnail.header { display: none; }
.single-project .post-thumbnail { display: none; }