[Opserty]

If I disable my plugins using the code "define('DISABLE_HOOKS', true);" then it disappears.

There is the important line! Did that not work?

(Remove that line if you haven't already done so).

So, now go through the Plugins in your Plugin manager (the ones related to this product) and disable them one by one till the copyright disappears. You seem to have found the problem in the XML (and when the xml file is imported this data is then stored in the Plugin Manager) so I'm not quite sure why you haven't figure this out so far.

P.S. From the XML file you should be able to find the Plugin name (it will be near the piece of code you posted in your original post). Else if you can find it just follow the instructions in the previous paragraph.