Assets: Replace duplicated asset library reference type from DNA

Since recently it's possible to access assets from outside the
File/Asset Browser, via the asset view template. So we are slowly
moving away from file space specific code to dedicated asset system
code. I introduced `AssetLibraryReference` as a duplicate of
`FileSelectAssetLibraryUID`, with a plan to delete the latter in a
separate cleanup commit. That's exactly what this commit is.

This will cause Asset Browsers to open with the default "Current File"
Asset Library. We could avoid that, but it's a minor issue really.

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