Remove the concept of Document activation callbacks
Since this was introduced, "ActiveDOMObject" has also been introduced, and it seems to be a more robust replacement for it.
At this point (102829) there's only *one* user of document activation left in SVG, so it'll be trivial to move it over to ActiveDOMObject.
Actually HTMLFormElement uses the activation callback too.
This would be nice to get to some day.
Per the discussion in https://bugs.webkit.org/show_bug.cgi?id=74533 ActiveDOMObject isn't actually a panacea.
So we're no longer sure we're going to do this.