On my page that needed tidying I have removed/disabled everything. Since these pages are for our use/reference, and not needing translation by others, I don't think there's any point in leaving any of it. Anyway, you can check the result of your edit on Special:PageTranslation. If it removes the page from the translatable pages, and asks if it should be removed from the translate system altogether, the job is done. It's safe to experiment.
I have now removed the page from the page translation system as you described. It worked without removing the auto-generated tags. (Actually it would have been easier to remove them along with the translate tags, but I was afraid that this would have messed up the translation system.)
I have also deleted the Danish translation.
That's interesting. I was asked to delete another translated page and could not. Maybe it's because you were the original author that you can do it. Did you have to do anything special?
No, I just deleted the < languages /> tag and all the < translate> and < /translate> tags. I only remove the page from the translation system and delete the Danish page the next day since I didn't know how at first, so maybe timing is an issue. Or maybe the English page has to be removed from the translation system before translated pages can be deleted?