cleanup: Use default parameters where appropriate

Since ES6 it is possible to set an explicit default value for optional
parameters (overriding the implicit value of 'undefined'). Use them
for a nice small cleanup.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/626

js/ui/osdWindow.js

@@ -151,9 +151,7 @@ var OsdWindow = class {
         }
     }
 
-    setMaxLevel(maxLevel) {
-        if (maxLevel === undefined)
-            maxLevel = 100;
+    setMaxLevel(maxLevel = 100) {
         this._level.maxLevel = maxLevel;
     }