Remove erroneous front matter field

Fixes gohugoio#2418

content/en/content-management/front-matter.md

@@ -131,9 +131,6 @@ videos
 weight
 : used for [ordering your content in lists][ordering]. Lower weight gets higher precedence. So content with lower weight will come first. If set, weights should be non-zero, as 0 is interpreted as an *unset* weight.
 
-taxonomies
-: Field name of the *plural* form of the index. See `tags` and `categories` in the above front matter examples. *Note that the plural form of user-defined taxonomies cannot be the same as any of the predefined front matter variables.*
-
 {{% note %}}
 If neither `slug` nor `url` is present and [permalinks are not configured otherwise in your site configuration file](/content-management/urls/#permalinks), Hugo will use the file name of your content to create the output URL. See [Content Organization](/content-management/organization) for an explanation of paths in Hugo and [URL Management](/content-management/urls/) for ways to customize Hugo's default behaviors.
 {{% /note %}}