[mkcal] Revert proper saving of dtEnd for alldays.
This is mostly a revert of acf7c0e9 to keep backward compatibility with already saved events in database with dtEnd + 1.
@chriadam I'm sorry this is another emergency fix. I've just noticed last week that some of my all day events where spanning on two days instead of one. I think the commit introducing the bug is not in 3.3.0, but it should not go in the next, otherwise we'll have events with dtEnd + 1 and some without. I'm sorry, I didn't think about backward compatibility with existing events in db when doing the erroneous commit.