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Fix T65054: Blender 2.80 crashes when opening a scene created by blender 2.7x with hierarchy duplication set to "group".

Regression introduced by rB7fe3d1e7d718 (fixing T57934).

This effectively reverts rBrB7fe3d1e7d718, since changing the type of an
object is a very bad idea in general, and would need a careful and
complex check of all of its usages (many object usages assume a specific
type of object...).

Instead, we simply remove duplicollection on those objects, and give a
warning in case dupligroup was effectively used (in many reported cases,
dupligroup was a 'mistake setting', without actually instancing
aniything).

Note that the otehr idea to fix that versioning issue (to create a new
empty object for the instancing) is much less easy than it might look
(one would need to take into account potential animations, relations
between objects, etc.). Doable probably, but way overkill for a
corner-case 'bad' usage of the feature in the first place.