libfdt: Remove current version

The current version of libfdt (1.4.2) has been modified to integrate it
in this repository. In order to do a clean import it is needed to remove
the current version first.

Change-Id: I2cab8c8e5632280d282fa7a2f2339768a0ad1e0f
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>