Don't prevent overflow on y-as (#19978)

Don't prevent overflow on y-as (#19978)

- Fomantic tries to prevent overflowing on the `y/x`-as by default on
stackable menu's on mobile screens. We already solve this issue by
forcing overflow on x as and hide it on y as(due to some issues with
other menu's), since https://github.com/go-gitea/gitea/pull/19486.
- However this edge case does require a y-overflow to show the dropdown,
because you cannot easily adjust this with CSS, once you're fiddling
with overflow's (https://stackoverflow.com/a/6433475). However
interesting behavior is noted
https://css-tricks.com/popping-hidden-overflow/ when you remove the
position: relative, it will suddenly work again. Well because this is
the only solution without redesigning dropdowns, I think we can live
with the side-effect of the dropdown items being full-width instead
"relative" width to their parent.
- Resolves #19976

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>