Remove the `load_folder` API

[alice-i-cecile]

Originally posted by @andriyDev in #19717 (comment):

@alice-i-cecile I am against the load_folder API. I think my ideal API is something like this:

struct LoadBatchRequest {
    requests: Vec<Glob>,
}

So you could do:

vec![
    "my_mesh.gltf",
    "animations/walk_*.png",
    "animations/run_*.png",
    "particles/*",
]

We can even make this itself an asset, so you'd just do asset_server.load::<LoadedBatch>("my_load_request.batch.ron");. I think this allows us to be more targeted and is generally more flexible. You can still do folder loads, but you can also target specific file paths.

I would prefer not to merge this.

[alice-i-cecile]

More discussion against building upon the load_folder API: #19729 (review)