Instead of adding the default "Bones" bone collection to every Armature
data-block, only add it for Armatures that are added in the 3D View with
Shift+A > Armature.
This simplifies things like importers, which otherwise would have needed
to remove that default collection before creating the imported ones.
Additionally, this also ensures that the one bone you get by default is
actually assigned to the default bone collection.
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