Blender Git Loki

Assets: Rename workspace active asset library DNA variable

This new variable was introduced with 7898089de3f2. We don't usually use
an `active` prefix variable. Plus, this makes the name match the one of
the Asset Browser active library variable, so we can use the
`rna_def_asset_library_reference_common()` helper for both.

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