Remove useless namespace declaration

content/transparent_reader_toolbar.css

@@ -3,8 +3,6 @@ See the above repository for updates as well as full license text. */
 
 /* Toolbar is invisible until hovered */
 
-@namespace html url("http://www.w3.org/1999/xhtml");
-
 @-moz-document url-prefix("about:reader"){
   .reader-toolbar, .reader-toolbar .button:not(:hover) {
     background-color: transparent !important;