Remove unused content.html

This is no longer used, now that we're overriding Docsy's `single.html` template directly.

Previously, this content view was rendered by Docsy's own `single.html`
template. But it's clearer to just override said template like we do
with `list.html` already.

layouts/content.html

@@ -1,3 +0,0 @@
-{{ define "main" }}
-  {{ partial "spec-content.html" . }}
-{{ end }}