The utility of the function is extremely limited as for block copy
we need to register the mirror chain earlier than when it's set with the
disk. This means that it would be open-coded in that case.

Avoid any weird usage and just open-code the only current usage, remove
the function, and reword the docs.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>