[MC-64379] Difficulty is still in options.txt

I am really sorry if this is a duplicate and I don't really know if this is really a bug or maybe intended

When you open your options.txt document, you see there still a difficulty tag even there is no "global" difficulty anymore.

It also seems like all the worlds you create have first the difficulty "Normal" regardless what value you have for the difficulty in the options.txt

But when you delete this tag it won't appear again so this is not a heavy bug