[migration/3.18] Explicitly delete constraint on defaultval
Make sure the SizeConstraint on defaultval is removed, and avoid more
unnecessary commits.
<div id="ajaxroot">
<div class="ajaxHtmlHead">
<script src="http://foo.js" type="text/javascript"> </script>
<link rel="stylesheet" type="text/css" media="all" href="qunit.css" />
</div>
<h1>Hello</h1>
</div>