The flag syncing code expects to find collection flags in same view
layer before and after the move, it even has an assert for it. However,
there is one case where this doesn't happen, when dragging a collection
that exists in two scenes to the master collection.
This commit removes this assert, frees the temporary flag structs
separately, and updates some comments with this information.
There is more detail in the adjusted comment.
Differential Revision: https://developer.blender.org/D9785
97651f428b