[epub] fix for calibre warning - cover does not need linear=no attribute

Calibre said „There are items marked as non-linear in the <spine>.
These will be displayed in random order by different ebook readers.
Some will ignore the non-linear attribute, some will display them at
the end or the beginning of the book and some will fail to display them
at all. Instead of using non-linear items simply place the items in the
order you want them to be displayed.”