Remove "top_graphic" article config item

[aarongable]

Many articles in this repo include a top_graphic: X in the configuration at the top of the file. For example:

website/content/en/about.md

Line 5 in fc52723

top_graphic: 1

This is processed by the templating system to determine which image to put in the article's header:

website/layouts/partials/hero.html

Line 2 in fc52723

{{ $bgImage := printf "images/%d.jpg" (.Param "top_graphic") | relURL }}

But, all of the "X.jpg" files in the //static/images directory are identical.

We should remove this config item, remove all of the redundant copies of the image, and hard-code hero.html to always reference a single image.