After replacing Lang.Class with ES6 classes and adopting arrow notation
for anonymous callbacks, we only use the Lang module to bind `this` to
named callbacks. However since ES5, this functionality is already provided
by Function.prototype.bind() - in fact, Lang.bind() itself uses it when
no extra arguments are specified.
So just use the built-in function directly instead of the wrapper.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/30
ba27cc4a64