Revert "Fix missing "pure-img" class on resource_image shortcode"

> This is not required - the theme does take care of it

This reverts commit 21bf40051d.

layouts/shortcodes/resource_image.html

@@ -8,5 +8,5 @@
 {{- end }}
 {{- $img := resources.Get $src | resources.Fingerprint }}
 <p>
-  <img class="pure-img centered pad-vertical" src="{{ $img.RelPermalink }}" alt="{{ $alt }}">
+  <img class="centered pad-vertical" src="{{ $img.RelPermalink }}" alt="{{ $alt }}">
 </p>