Revert "fdt- Tell the FDT library where the device tree is"

This reverts commit 3b73459e.

In practice it doesn't seem like a good idea to make the the working
FDT point to the control FDT. Now that we can access the control FDT
using the 'fdt' command, there is no need for this feature. Remove it.

Signed-off-by: Simon Glass <sjg@chromium.org>