How to remove featured image from single page?

That isn’t possible with the available options in the free edition of the theme. The Plus edition has additional meta options for that.

With the free edition it can still be done with CSS:

.page-id-123 #masthead #header-image-main .header-image {
    display: none;
}

replace 123 with the correct ID number of your target page.

This CSS cannot be used with the menu over header image option (since there will be no header image for the menu to sit on, making it overlap the content instead).