You could delete it from the skin flies.
<div id="composeoptionsmenu" class="popupmenu"> <table><!-- <tr> <td><label for="rcmcomposereceipt"><roundcube:label name="returnreceipt" />:</label></td> <td><roundcube:object name="receiptCheckBox" form="form" id="rcmcomposereceipt" /></td> </tr> --> <tr> <td><label for="rcmcomposepriority"><roundcube:label name="priority" />:</label></td> <td><roundcube:object name="prioritySelector" form="form" id="rcmcomposepriority" /></td> </tr><tr> <td><label><roundcube:label name="savesentmessagein" />:</label></td> <td><roundcube:object name="storetarget" maxlength="30" /></td> </tr> </table></div>