[Low] Tutorials: "backlink" front matter variable seems unused

### Resource URL

https://www.w3.org/WAI/tutorials/images/examples/2014-first-qtr/

### Description

Some pages include a `backlink` front matter variable.

```
backlink: /tutorials/images/complex/#image-containing-substantial-information
```

This variable seems unused and could probably be removed.

### Resource Shortname

wai-tutorials