docs: Remove unused #sponsor CSS rule

Commit e14c5069 dropped the only user of the 'sponsor'
CSS id, so we can drop it from the stylesheet as well.

docs/libvirt.css

@@ -300,12 +300,6 @@ p.image {
     color: #ccc;
 }
 
-#sponsor {
-    color: #757575;
-    text-decoration: inherit;
-    font-size: 1.2em;
-}
-
 span.since {
     color: #3c857c;
     font-style: italic;