Add `AssetServer::remove`

## What problem does this solve or what need does it fill?

It is sometimes useful to manually unload assets, without having to track down and drop every existing handle. You can currently do this with `Assets::remove`, but no equivalent method on `AssetServer` exists.

## What solution would you like?

Add `AssetServer::remove`.

## What alternative(s) have you considered?

Continue to use the workaround. This won't play nice with heterogenous asset collections, as you need to know the type of the asset.

## Additional context

> Oh wow, i completely missed that thinking it would be in AssetServer 😆 thank you!

Prompted by [Carl on Discord](https://discord.com/channels/691052431525675048/742569353878437978/1219465980250427412).

Start by reading the existing Assets::remove API and the AssetServer API entry points to understand how asset removal is currently exposed. Add the requested AssetServer::remove equivalent and verify that it supports unloading assets without requiring callers to track every handle or know the asset type.

Written by the indexing model from the issue text.