client/general: remove 404 image

It used to be relevant when we had Tsukasa for mascot, but since the 2.x
strives to look more "professional" and there's no Tsukasa in the
README, it just looks out of place.

client/html/not_found.tpl

@@ -1,5 +1,5 @@
 <div class='not-found'>
-    <img src='/img/404.png' alt='404 Not found'/>
+    <h1>Not found</h1>
     <p><%- ctx.path %> is not a valid URL.</p>
     <p><a href='/'>Back to main page</a></p>
 </div>