js: Avoid double declarations with let

The following code is a syntax error in ES6:

    let a = 'something';
    let a = 'other thing';

Previously GJS would silently accept this code, but in the next release the
SpiderMonkey JS engine will be more ES6-compliant.

https://bugzilla.gnome.org/show_bug.cgi?id=778425

js/ui/dateMenu.js

@@ -77,7 +77,7 @@ const TodayButton = new Lang.Class({
          * below the time in the shell; it should combine the weekday and the
          * date, e.g. "Tuesday February 17 2015".
          */
-        let dateFormat = Shell.util_translate_time_string (N_("%A %B %e %Y"));
+        dateFormat = Shell.util_translate_time_string (N_("%A %B %e %Y"));
         this.actor.accessible_name = date.toLocaleFormat(dateFormat);
     }
 });