Update prototype.js to version 1.6.1

[Bernhard Reiter]

prototype.js version 1.6.1 was released on Sept 1, 2009, and includes a couple of new features useful to plugin and theme authors, so please update the included javascript!

See ​http://prototypejs.org/2009/9/1/prototype-1-6-1-released

[azaozz]

It's too late for this to go in 2.9 as the updated Prototype may cause compatibility problems with some plugins.

[dancole]

Simek submitted a patch at #11594, attachment: ​porototype.update.to.1.6.1.patch

[azaozz]

(In [12557]) Update Prototype.js to 1.6.1 and remove the second copy of it from wp-includes/js/scriptaculous, update Scriptaculous.js to 1.8.3, props Simek, fixes #11041