Remove encoding attribute which seems to not actually exist

AsciiDoc processors should already use UTF-8 by default, according to
the FAQ:
https://docs.asciidoctor.org/asciidoc/latest/faq/#does-asciidoc-only-support-ascii-text

src/docs/asciidoc/prob_handbook.adoc

@@ -3,7 +3,6 @@ Jens Bendisposto;Joy Clark;Michael Leuschel
 :bibtex-file: combined.bib
 :coderay-linenums-mode: table
 :doctype: book
-:encoding: utf-8
 :icon: font
 :imagesdir: images
 :linkattrs: