Please note, Geany isn't Vim, IDEA, Eclipse or any other editor.
That binding is undocumented so it could do anything, including eating your shorts. And it does not have a menu entry. And in fact it does `join_paragraph()` so its also misnamed. So in general that entry should be deleted. Especially as joining a paragraph has been superseded by reflow lines.
After its deleted someone could replace it with some other functionality, like simple join lines, and this time with a menu entry as well. Just needs somebody to make a pull request.