The `attrHooks` entries for boolean attributes are only defined for jQuery 4+;
jQuery 3.x used a separate mechanism - assigning them to
`jQuery.expr.attrHandle`. That object used to be maintained by Sizzle, since
jQuery 3.7.0 it's kept in the selector module. Because of that, the `isXMLDoc`
check used to be require in this hook.
Now that standard `attrHooks` are used, the `isXMLDoc` check already happens
inside of `jQuery.attr` and there's no need to repeat it in the test. Note that
this repetition is even incorrect - while Sizzle's `jQuery.find.attr` used to
treat an `undefined` output of the hooks from `jQuery.expr.attrHandle` as a way
to opt out of the hook, jQuery's `attrHooks` use `null` to opt out of a getter
hook.
Apart from the size, this patch also avoids unnecessary extra checks.
Closes gh-5398
b40a4807b6