Stop adding negative leveloffset for EPUB

asciidoctor-epub3 1.5.0-alpha-14 now handles whole book as a single document.

Negative leveloffset
1. Breaks appendix because appendix subsections are not allowed to be higher that level 3. When leveloffset is applied to them, they become level 2 and are no longer children of appendix and instead become standalone chapters
2. Transforms progit into a multi-part book instead of multi-chapter book, what is wrong

After removal of negative offset, epub is structurally equivalent to other formats.

Author: slonopotamus

progit.asc

@@ -8,8 +8,6 @@ Scott Chacon; Ben Straub
 :front-cover-image: image:book/cover.png[width=1050,height=1600]
 :icons: font
 
-ifdef::ebook-format[:leveloffset: -1]
-
 include::book/license.asc[]
 
 include::book/preface_schacon.asc[]