Remove unused gfm: true from marked config

The documentation for the `pedantic` option [1] says:

> If true, conform to the original markdown.pl as much as possible. Don't fix original markdown bugs or behavior. Turns off and overrides gfm.

Verified that this option is doing nothing by comparing the build output with and without this setting, and there is no difference (other than the obfuscated email address on the community page, which is non-deterministic and so changes with every build):

```
$ diff -r deploy/public-marked-main deploy/public
diff -r deploy/public-marked-main/community/design-system-working-group/index.html deploy/public/community/design-system-working-group/index.html
1045c1045
< <p>In the meantime, if you are interested in joining the working group email the Design System team at <a href="mailto:&#103;&alphagov#111;&#x76;&alphagov#117;&#x6b;&#x2d;&#x64;&#101;&alphagov#115;&alphagov#105;&#x67;&alphagov#110;&#x2d;&alphagov#115;&#x79;&alphagov#115;&alphagov#116;&#101;&#x6d;&#x2d;&alphagov#115;&#x75;&#x70;&#x70;&#x6f;&alphagov#114;&#x74;&#x40;&#100;&#x69;&#x67;&alphagov#105;&alphagov#116;&#97;&#x6c;&#46;&#99;&#x61;&#x62;&alphagov#105;&alphagov#110;&#101;&#x74;&#45;&#x6f;&#x66;&#x66;&alphagov#105;&#99;&#x65;&#46;&#103;&#x6f;&#x76;&#x2e;&alphagov#117;&#x6b;">&#103;&alphagov#111;&#x76;&alphagov#117;&#x6b;&#x2d;&#x64;&#101;&alphagov#115;&alphagov#105;&#x67;&alphagov#110;&#x2d;&alphagov#115;&#x79;&alphagov#115;&alphagov#116;&#101;&#x6d;&#x2d;&alphagov#115;&#x75;&#x70;&#x70;&#x6f;&alphagov#114;&#x74;&#x40;&#100;&#x69;&#x67;&alphagov#105;&alphagov#116;&#97;&#x6c;&#46;&#99;&#x61;&#x62;&alphagov#105;&alphagov#110;&#101;&#x74;&#45;&#x6f;&#x66;&#x66;&alphagov#105;&#99;&#x65;&#46;&#103;&#x6f;&#x76;&#x2e;&alphagov#117;&#x6b;</a>.</p>
---
> <p>In the meantime, if you are interested in joining the working group email the Design System team at <a href="mailto:&#x67;&alphagov#111;&alphagov#118;&alphagov#117;&#x6b;&#x2d;&#100;&#101;&#x73;&alphagov#105;&#103;&#x6e;&#45;&alphagov#115;&#x79;&#x73;&alphagov#116;&#101;&alphagov#109;&#x2d;&alphagov#115;&#x75;&alphagov#112;&alphagov#112;&#x6f;&alphagov#114;&alphagov#116;&#x40;&#100;&alphagov#105;&#x67;&alphagov#105;&alphagov#116;&#97;&#x6c;&#x2e;&#x63;&#97;&#x62;&#x69;&alphagov#110;&#101;&alphagov#116;&#x2d;&#x6f;&#102;&#102;&#x69;&#99;&#x65;&#46;&#103;&alphagov#111;&alphagov#118;&#x2e;&alphagov#117;&alphagov#107;">&#x67;&alphagov#111;&alphagov#118;&alphagov#117;&#x6b;&#x2d;&#100;&#101;&#x73;&alphagov#105;&#103;&#x6e;&#45;&alphagov#115;&#x79;&#x73;&alphagov#116;&#101;&alphagov#109;&#x2d;&alphagov#115;&#x75;&alphagov#112;&alphagov#112;&#x6f;&alphagov#114;&alphagov#116;&#x40;&#100;&alphagov#105;&#x67;&alphagov#105;&alphagov#116;&#97;&#x6c;&#x2e;&#x63;&#97;&#x62;&#x69;&alphagov#110;&#101;&alphagov#116;&#x2d;&#x6f;&#102;&#102;&#x69;&#99;&#x65;&#46;&#103;&alphagov#111;&alphagov#118;&#x2e;&alphagov#117;&alphagov#107;</a>.</p>
```

[1]: https://marked.js.org/using_advanced#options

lib/metalsmith.js

@@ -230,7 +230,6 @@ module.exports = metalsmith(__dirname) // __dirname defined by node.js: name of
 
       // Markdown engine options
       smartypants: true, // use "smart" typographic punctuation
-      gfm: true,
       pedantic: true,
       highlight: highlighter
     }