Update list and single layouts for Hugo 0.63.1

The changes to the base template lookup in Hugo 0.63.1 mean that the
single/list templates must start with a `define` block. The comment
block that had been in use breaks this requirement and resulted in pages
not rendering properly at all.

See https://gohugo.io/news/0.63.0-relnotes/ particularly:

* Note: In a base template (e.g. baseof.html), the first template block
must be a define.

layouts/_default/list.html

@@ -1,4 +1,3 @@
-{{/* This will override the default value set in baseof.html; i.e., "{{.Site.Title}}" in the original example */}}
 {{ define "title" }}
 {{ .Title }} – {{ .Site.Title }}
 {{ end }}

layouts/_default/single.html

@@ -1,4 +1,3 @@
-{{/* This will override the default value set in baseof.html; i.e., "{{.Site.Title}}" in the original example */}}
 {{ define "title" }}
 {{ .Title }} – {{ .Site.Title }}
 {{ end }}