layouts/_default/single.html: Improve Schema.org keywords

Use delimit function over a union of Categories and Tags. This is
much easier to read, results in correct delimiting with no trailing
comma, and works even if one of the slices is nil.

layouts/_default/single.html

@@ -41,7 +41,6 @@
   }
   {{- if or (.Params.categories) (.Params.tags) -}}
   ,
-  {{- /* delimit keywords by comma, even if one of .Params.categories or Params.tags is nil */ -}}
   "keywords": "{{ delimit (union .Params.categories .Params.tags) ", " }}"
   {{- end }}