tools/fit_image.c: Remove unused fit_set_header()

The FIT fit_set_header() function was copied from the standard uImage's
image_set_header() function during mkimage reorganization.  However, the
fit_set_header() function is not used since FIT images use a standard
device tree blob header.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>