js: Use SOURCE_CONTINUE/SOURCE_REMOVE constants in source functions

With support for boolean constants in g-i, we can finally use the
more readable constants instead of true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=719567

js/gdm/util.js

@@ -250,6 +250,7 @@ const ShellUserVerifier = new Lang.Class({
                                                        Lang.bind(this, function() {
                                                            this._messageQueueTimeoutId = 0;
                                                            this._queueMessageTimeout();
+                                                           return GLib.SOURCE_REMOVE;
                                                        }));
     },