Remove capture shortcode

The capture shortcode is no longer used: the switch was made on
2020-06-15, in commit 0625ced466.

Remove it. After this change, it won't be possible to use this legacy
shortcode (builds would fail).

layouts/shortcodes/capture.html

@@ -1,8 +0,0 @@
-{{ $_hugo_config := `{ "version": 1 }`}}
-{{- $id := .Get 0 -}}
-{{- if not $id -}}
-{{- errorf "missing id in capture" -}}
-{{- end -}}
-{{- $capture_id := printf "capture %s" $id -}}
-{{- .Page.Scratch.Set $capture_id .Inner -}}
-{{ warnf "Invalid shortcode: %s, in %q" $capture_id (relLangURL .Page.Path) }}