Layout tutorial: remove unused Liquid variable (#2293)

Author: chalin

src/docs/development/ui/layout/tutorial.md

@@ -6,7 +6,6 @@ diff2html: true
 ---
 
 {% assign api = 'https://docs.flutter.io/flutter' -%}
-{% capture code -%} {{site.repo.this}}/tree/{{site.branch}}/src/_includes/code {%- endcapture -%}
 {% capture examples -%} {{site.repo.this}}/tree/{{site.branch}}/examples {%- endcapture -%}
 {% assign rawExFile = 'https://github.com/raw/flutter/website/master/examples' -%}
 {% capture demo -%} {{site.repo.flutter}}/tree/{{site.branch}}/examples/flutter_gallery/lib/demo {%- endcapture -%}