To make the faux spine disappear, you simply have to add this line to the
<meta>section of your opf file (in an EPUB 3 fixed layout file):
And as long as you have version 3.1 or later of iBooks, the faux binding (and the faux pages at the edges) disappear:
Note, however, that if you designed your pages to fit perfectly in a landscape-oriented iOS device, the disappearance of the faux pages will make your book slightly more narrow, and you will have some empty gray space as shown above.
(Thanks to Jean-Michel Dentand for reminder to update to new version of iBooks!)