of: unittest: Remove redundant OF_DETACHED flag setting

of_fdt_unflatten_tree() already sets the flag on the node to
OF_DETACHED, because of_fdt_unflatten_tree() calls
__unflatten_device_tree() with the detached bool set to true.

Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>