remove chapter-signifier and part-signifier from theme-related attributes table [skip ci]
remove chapter-signifier and part-signifier from theme-related attributes table [skip ci]

these attributes are now part of AsciiDoc itself

mojavelinux/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,asciidoctor/asciidoctor-pdf