[javascript] remove ajaxHtmlHead from dom response once it has been processed.
This fixes #549138: treeview: folding is broken
The new, systematic, pageid management (introduced by rev 37b376bb4088) made
the treeview return :
<ul class="treeview">...</ul>
<div class="ajaxHtmlHead">...</div>
which was then processed by getDomFromResponse and wrapped into
a single div node :
<div>
<ul class="treeview">...</ul>
<div class="ajaxHtmlHead">...</div>
</div>
In the treeview case, the node inserted into the tree was the wrapping
<div> instead of the <ul>, causing the folding bug mentioned in the
ticket.
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