Hide Featured Image in Post in Twenty Fifteen

I just had the same – or actually rather a similar problem. I wanted posts with and without FI to have the same top padding. I hope your problem has the same cause so that the following css could also work for your:

In Twenty Fifteen Parent style.css line 4448 the following is defined:

.hentry {
    padding-top: 8.3333%;
}

but in line 1582 however posts with a featured image are treated differently:

.hentry.has-post-thumbnail {
    padding-top: 0;
}

So in my child theme style.css I just applied the same top padding to both cases like this:

.hentry, 
.hentry.has-post-thumbnail {
    padding-top: 8.3333%;
}

I hope that will point you in the right direction