Really unnecessary now.

This reverts a portion of ppy#9539.

The rearrangement in `SongSelect` is required to get the initial filter
into `BeatmapCarousel` (and avoid the `FilterChanged` event firing,
causing a delayed/scheduled filter application).

The only exception to the rule here was "when screen isn't active apply
without debounce" but I'm not sure we want this. It would cause a
stutter on returning to song select and I'm not even sure this is a
common scenario.

I'd rather remove it and see if someone finds an actual case where this
is an issue.

…ng implicit

These used to work because there was a huge blocking load operation,
which is now more asynchronous.

Note that the change made in `SongSelect` is not required, but defensive
(feels it should have been doing this the whole time).

The explicit detach call was removed from `updateBeatmapSet`, causing
this to occur. We could optionally add it back (it will be a noop in all
cases though).