fix(build): remove duplicated Signature from the 'signArchives' task
fix(build): remove duplicated Signature from the 'signArchives' task

The extra Signature went unnoticed until Gradle 5.1 improved up-to-date
checking for the Sign task, and revealed the duplicate; the build breaks
when checking whether it should skip ':asciidoctorj-distribution:signArchives'

abelsromero/asciidoctorj,robertpanzer/asciidoctorj,abelsromero/asciidoctorj,robertpanzer/asciidoctorj,asciidoctor/asciidoctorj,abelsromero/asciidoctorj,robertpanzer/asciidoctorj,asciidoctor/asciidoctorj,asciidoctor/asciidoctorj