The "uninstallcode" section of the product XML is blank. That means that when uninstalling, the product's SQL changes are left behind. This is rather poor practice. Can it be updated? Seems like all you'd need is to drop the "sktimer" table.
Code:
<installcode><![CDATA[$vbulletin->db->query("CREATE TABLE IF NOT EXISTS " . TABLE_PREFIX . "sktimer (
skuserid int(11) NOT NULL default '0',
sktitle text,
skdescription text,
skdateline int(11) default NULL,
skactive tinyint(4) default NULL,
skformat enum('dhms','dh') default 'dhms',
PRIMARY KEY (skuserid)
)");]]></installcode>
<uninstallcode />
Edit: The updated product XML in my post above includes uninstall code which drops the "sktimer" table.