Reported at https://osu.ppy.sh/community/forums/topics/2015478?n=1.
Would you believe it that this button that has been there for literal
years never did anything?
Implemented at a per-beatmap level. Also additionally added to context
menu (at @peppy's suggestion), and also copy reworded from "Delete from
unplayed" to "Mark as played" because double negation hurt my tiny
brain.
The whole component is pointless so I'm just going to nuke for now I
guess. Kind of makes the whole refactor effort pointless but oh well?
To expand on this, the implementation was actually incorrect as pointed
out at https://github.com/ppy/osu/pull/18819#pullrequestreview-1017886035.
Even with pooling applied, there are overheads involved with transforms
when quickly cycling through the carousel.
The main goal here is to reduce the transforms in cases the reuse is
still in the same state. Avoiding firing `FadeIn` and `FadeOut` are the
main areas of saving.