Blender Git Loki

Library Overrides: Unlink instance empty after "Make Library Override"

Unlink (delete if single-user) collection instance empty once an override is
added to the collection through the "Make Library Override" operator. It isn't
used as a collection instance anymore then so the emtpy is an annoying left
over that has no purpose.

Part of T76555.

Reviewed by: Andy Goralczyk, Bastien Montange.

Differential Revision: https://developer.blender.org/D7626