Always mute timestamp from screen readers

The short time is still read out (eg: 15:24), however by ignoring the anchor we prevent the reader from saying the title of the containing span. This prevents readers saying "Wed May 22, 2019 at 15:24 15:24".

See element-hq/element-web#9747

src/components/views/rooms/EventTile.js

@@ -789,7 +789,7 @@ module.exports = withMatrixClient(React.createClass({
                         </div>
                         { sender }
                         <div className="mx_EventTile_line">
-                            <a href={permalink} onClick={this.onPermalinkClicked} aria-hidden={muteScreenReader}>
+                            <a href={permalink} onClick={this.onPermalinkClicked} aria-hidden={true}>
                                 { timestamp }
                             </a>
                             { this._renderE2EPadlock() }