[dtc] Get rid of libdt.c

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> libdt.c was an attempt at creating a device tree handling library
> within the dtc codebase.  However, it was never even close to
> completion, and is entirely obsoleted by it's spiritual descendent,
> libfdt (currently a separate package).  This patch, therefore, removes
> libdt.c entirely, along with its only reference in the Makefile, an
> unused variable.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl