Bug 1780017 - Remove undefined export from UrlbarPrefs.jsm. r=adw

Differential Revision: https://phabricator.services.mozilla.com/D152055

browser/components/urlbar/UrlbarPrefs.jsm

@@ -10,7 +10,7 @@
  * preferences, but only for variables with fallback prefs.
  */
 
-var EXPORTED_SYMBOLS = ["UrlbarPrefs", "UrlbarPrefsObserver"];
+var EXPORTED_SYMBOLS = ["UrlbarPrefs"];
 
 const { XPCOMUtils } = ChromeUtils.importESModule(
   "resource://gre/modules/XPCOMUtils.sys.mjs"