Change the way remove children is called in bdev

For some cases, we don't have to have access to the children of a device
in order to remove them (e.g. md over lvs, or drbd over lvs). In order
to ease the removal process, skip over finding the child if it provides
a static dev path.

This is needed in order to support removal of children when the
underlying storage has gone away.

Reviewed-by: imsnah