Seems to be residue from an early 2.80 days: the placeholder code path
is no longer used.
Remove all the tricky code for this, and make it clear that the
`deg_expand_copy_on_write_datablock` is used on an non-expanded
datablock.
Should be no functional changes. And should help simplify D12850.
Differential Revision: https://developer.blender.org/D12852
c29f20a521