fix undefined variable included_syntaxes

Author: devsunb

autoload/d2.vim

@@ -128,13 +128,11 @@ function! d2#sync_markdown_fenced_languages() abort
 endfunction
 
 function! d2#syntax_post() abort
-  if index(b:included_syntaxes, 'markdown') != -1
-    if hlexists('markdownCodeBlock')
-      syn clear markdownCodeBlock
-    endif
-
-    syn cluster d2BlockStringMarkdown add=@markdownBlock
+  if hlexists('markdownCodeBlock')
+    syn clear markdownCodeBlock
   endif
+
+  syn cluster d2BlockStringMarkdown add=@markdownBlock
 endfunction
 
 " include_toplevel_markdown is called when markdown is the top level syntax and is