Remove quote-block transcoder function

The only reason for using this function is to add reveal flag classes. However, these are already added as classes to :attr_html when the parse tree is filtered: org-re-reveal-filter-parse-tree, and org-re-reveal-append-frag. Removing the transcoder function enables other attributes (like cite) to be added with an #+attr_html: line, which will be parsed by org-html-quote-block.