The error you're receiving is due to the fact that the element you're trying to delete does not have the "change attribute" support to allow CPA to track a change on the element, including touchup 'commands.' It's true that you could modify the DTD to add the various attributes to support CPA so that it would allow the deleting of the tag (your _touchup 'command'), however not advised in this case.
An easier way to do this is to not tell CPA you're deleting the tag. Temporarily turn Change Tracking off for the session by pressing <ctrl+shift+e>. On the bottom right of the screen, you'll see "TRK" now grayed out. When Change Tracking is off, anything you do to the document is not tracked as a change, nor is it evaluated by CPA to be a change. This should allow you to delete the _touchup command without CPA yelling at you. When done, ensure you turn Change Tracking back on by once again hitting <ctrl+shift+e>. The "TRK" should now by dark implying it's turned on and tracking your changes.
A caveat to what I said about not tracking changes... When you create your change package, any change to a page will cause the page itself to be marked as a "changed" page. There are two parts to marking a page as changed; 1) By adding or deleting text with change markup, 2) A character somewhere on the page has changed whether done by some change in text or done by stylesheet. However, that's getting deeper than your question.
Hope this helps,