On more recent versions of setuptools the warning about not being able
to find the files specified in license_files has re-appeared. This is
because as best I can tell, it can't and won't look in $(srctree) but
rather only subdirectories of scripts/dtc/pylibfdt. Since we already
provide both SPDX tags and a license field with the SPDX contents, let
us just drop license_files as it's not mandatory.
Signed-off-by: Tom Rini <trini@konsulko.com>