Modification:
In private.php, find:
else
{
if($mybb->user['signature'] != "")
{
$optionschecked['signature'] = "checked";
}
if($mybb->usergroup['cantrackpms'] == "yes")
{
$optionschecked['readreceipt'] = "checked";
}
$optionschecked['savecopy'] = "checked";
}
And remove this line:
$optionschecked['savecopy'] = "checked";
Alternatively you can remove the $optionschecked[savecopy] variable from the private_send template, but you will lose the functionality of the checkbox selection being "remembered" when a user previews his or her PM.