You can not remove the title but you can set its width to 0. Please try this code:
<div id="aa" class="easyui-accordion" halign="left" style="width:700px;height:300px;">
<div title="Panel1" titleDirection="up" style="border:0">
<header style="width:0;padding:0;border:0"></header>
<p>Accordion Panel1.</p>
</div>
<div title="Panel2" titleDirection="up" style="border:0">
<header style="width:0;padding:0;border:0"></header>
<p>Accordion Panel2.</p>
</div>
<div title="Panel3" titleDirection="up" style="border:0">
<header style="width:0;padding:0;border:0"></header>
<p>Accordion Panel3.</p>
</div>
</div>