Remove OutputPage::{get,set}OnloadHandler()

Remove OutputPage::{get,set}OnloadHandler()

Didn't work at all in Monobook, Modern, Vector, or any derived skin, for
who knows how long.  Only one caller, which was using it to autofocus,
which is a bad idea anyway because it will cause the focus to happen too
late, often after the user has already interacted with the page.  Any
legitimate callers may as well just add the onload in pure JS without
adding an attribute to the body.